目錄
In Japanese grammar, 「ことにする」 and 「ことになる」 are often loosely translated into Chinese with the same word, “decide,” but in reality, these two patterns express completely different concepts. The difference comes down to who actually makes the final decision. If something is decided subjectively by the speaker, 「ことにする」 is used. If it is an objective result determined by someone else, a company or organization, or the way circumstances develop, 「ことになる」 is required. The two patterns look extremely similar, and when you open a textbook, the only visible difference is whether the final verb is 「する」 or 「なる」. That is why almost everyone studying at around the N4 level has struggled with them at some point. More intriguingly, even when Japanese speakers have actually made a decision themselves, they sometimes deliberately say 「ことになる」 in conversation. So how exactly can you distinguish this subtle gap in nuance? This article will clarify it all at once.
Why do Japanese learners need to understand 「ことにする」 and 「ことになる」?
If you do not understand the difference between 「ことにする」 and 「ことになる」, the most immediate consequence is that the nuance of what you say will be completely off, and the listener may misunderstand who made the decision. These patterns are a core N4 grammar point and appear extremely frequently in everyday conversation. Yet they are especially easy for native Chinese speakers to misuse because the Chinese word for “decide” does not distinguish between subjective and objective decision-making; one word covers both.
Consider a very common situation in Japanese dramas. If a character says 「東京へ転勤することになりました」 and you interpret it as “I decided to transfer to Tokyo,” you have completely misread the nuance. The point of the sentence is, “This wasn’t something I got to decide; the company is transferring me there,” so it carries a slight sense of having little control over the situation. By contrast, if the character says 「会社を辞めることにしました」, that is a firm declaration that “I decided to quit the company myself,” expressing an active choice backed by personal intention.
The distinction is even clearer in the workplace. When discussing business with a Japanese partner, if the other person says 「弊社で検討させていただくことになりました」, they are politely conveying something like, “After going through our internal process, this is the outcome,” placing responsibility for the decision on the organization. It sounds softer and more appropriately measured. If they instead say 「検討することにしました」, it can sound as though a particular individual personally made the call, reducing the sense of organizational distance and restraint. This kind of subtle nuance is at the heart of Japanese indirect expression.
Basic usage of 「ことにする」: expressing your own subjective decision
「ことにする」 is used to express a decision the speaker makes through their own will. In Chinese, it corresponds to “decide to do ~.” The key is that the verb 「する」 itself carries a sense of actively doing or exerting agency, so the focus of the entire pattern is always “I chose this myself.”
Conjugation rules for 「ことにする」
The attachment pattern for 「ことにする」 is simple: attach it directly to the dictionary form or ない-form of a verb. Because this pattern emphasizes voluntary intention, it cannot be attached to potential or passive forms. If you want to review verb conjugation first, you can go back to how the three verb groups are classified before returning to this pattern.
| Form | Pattern | Example |
| Verb (affirmative) | dictionary form＋ことにする | 行く → 行くことにする |
| Verb (negative) | ない-form＋ことにする | 行かない → 行かないことにする |
Usage 1 of 「ことにする」｜A decision made at that moment
When the speaker makes a decision at the moment of speaking, the past form 「ことにした」 is the most natural choice, meaning “I decided.” This usage most clearly highlights that “I was the one who made the decision,” and it often carries the tone of having made up one’s mind and then informing the other person of the result.
- Example: 今日からお酒を飲まないことにした。
Kana: きょうからおさけをのまないことにした。
Translation: I decided not to drink alcohol starting today.
This is a typical declaration of personal resolve. It emphasizes that giving up alcohol was the speaker’s own active choice rather than something forced on them, and it carries a sense of determination to follow through. - Example: いろいろ考えて、会社を辞めることにしました。
Kana: いろいろかんがえて、かいしゃをやめることにしました。
Translation: After thinking it over carefully, I decided to quit my job.
The phrase 「いろいろ考えて」 makes it clear that the decision was reached after considerable thought, while 「ことにしました」 clearly shows that the speaker ultimately made the choice themselves. - Example: 迷ったけど、留学することにした。
Kana: まよったけど、りゅうがくすることにした。
Translation: I hesitated, but I decided to study abroad.
「迷ったけど」 highlights the hesitation before the decision, while ending with 「ことにした」 brings out the final, decisive choice. This is also a very conversational way of saying it.
Usage 2 of 「ことにする」｜A habit or rule you decide for yourself
When 「ことにする」 becomes the ている-form 「ことにしている」, it expresses “a habit or rule that you decided on yourself and continue to follow.” The key is that this is a rule you set for yourself, not something everyone does as a matter of general social convention. For example, something like “greet your friends when you see them” normally would not use this pattern, because that is a general social norm rather than a special personal decision.
- Example: 毎朝、5時に起きることにしている。
Kana: まいあさ、ごじにおきることにしている。
Translation: I make it a habit to get up at 5 every morning.
This is a rule the speaker has set for themselves. 「ことにしている」 emphasizes that it is an intentionally maintained personal habit rather than something that simply happens naturally. - Example: 健康のために、寝る前にストレッチをすることにしている。
Kana: けんこうのために、ねるまえにストレッチをすることにしている。
Translation: For my health, I make it a habit to stretch before bed.
「健康のために」 explains the motivation, while 「ことにしている」 shows that this is a self-imposed habit the speaker continues in order to achieve a goal. - Example: 日記は日本語で書くことにしている。
Kana: にっきはにほんごでかくことにしている。
Translation: I always make a point of writing my diary in Japanese.
The speaker has made “writing the diary in Japanese” a personal rule, and 「ことにしている」 conveys the sense of maintaining that rule consistently.
Basic usage of 「ことになる」: expressing an objective decision or natural outcome
「ことになる」 is used when the result was not decided by the speaker personally, but instead came about because of another person, an organization, a rule, or the way circumstances developed. In Chinese, it corresponds to meanings such as “it turns out that ~” or “it has been arranged that ~.” The key is that 「なる」 means “to become” or “to change into,” so the focus is on “things ending up this way,” with the speaker more often positioned as someone who accepts the result.
Conjugation rules for 「ことになる」
The attachment pattern for 「ことになる」 is the same as for 「ことにする」: it follows the dictionary form or ない-form of a verb. The only difference is whether the final verb is 「なる」 or 「する」.
| Form | Pattern | Example |
| Verb (affirmative) | dictionary form＋ことになる | 行く → 行くことになる |
| Verb (negative) | ない-form＋ことになる | 行かない → 行かないことになる |
Usage 1 of 「ことになる」｜A decision made by another person or organization
When the decision is made by a company, school, another person, or some other party besides the speaker, the past form 「ことになった」 is used. This usage most clearly conveys “I did not have the authority to decide; it was arranged for me,” and it frequently appears in situations involving job transfers, assigned duties, and similar circumstances.
- Example: 来月から東京へ転勤することになりました。
Kana: らいげつからとうきょうへてんきんすることになりました。
Translation: I’ll be transferred to Tokyo starting next month.
The transfer is the company’s decision, so 「ことになりました」 clearly communicates that this was not the speaker’s own choice and carries the nuance of accepting an arrangement. - Example: 田中さんが休みなので、私が発表することになった。
Kana: たなかさんがやすみなので、わたしがはっぴょうすることになった。
Translation: Tanaka is off, so it ended up that I would give the presentation.
“Tanaka being absent” is an external factor that leads to the result “I will present.” The speaker is being pushed along by circumstances rather than actively choosing the outcome. - Example: 話し合いの結果、来年結婚することになりました。
Kana: はなしあいのけっか、らいねんけっこんすることになりました。
Translation: After discussing it, we decided that we would get married next year.
Marriage is of course something the two people decide together, but using 「ことになりました」 presents it as “the result that naturally emerged after discussion,” making the statement sound modest and understated.
Usage 2 of 「ことになる」｜Rules, conventions, or established arrangements
When 「ことになる」 becomes 「ことになっている」, it expresses rules, laws, conventions, or arrangements that have already been decided. In Chinese, it corresponds to meanings such as “the rule is that ~” or “it is scheduled that ~.” The key point is that this has already been determined and is not something an individual can change according to personal preference.
- Example: 日本では、車は道路の左側を走ることになっている。
Kana: にほんでは、くるまはどうろのひだりがわをはしることになっている。
Translation: In Japan, cars are required to drive on the left side of the road.
This is a traffic regulation and therefore an established rule, so 「ことになっている」 is used rather than 「ことにしている」. - Example: この寮では、12時までに帰らないといけないことになっている。
Kana: このりょうでは、じゅうにじまでにかえらないといけないことになっている。
Translation: This dormitory has a rule that residents must return by midnight.
The dormitory curfew is a rule set by the institution, so 「ことになっている」 indicates an externally imposed established regulation. - Example: 来週の金曜日、取引先と打ち合わせをすることになっている。
Kana: らいしゅうのきんようび、とりひきさきとうちあわせをすることになっている。
Translation: A meeting with a client is scheduled for next Friday.
Here, the sentence refers to an arrangement that has already been made. 「ことになっている」 emphasizes that the schedule is fixed and things are expected to proceed accordingly.
「ことにする」 vs 「ことになる」: the difference in nuance at a glance
The most important difference between 「ことにする」 and 「ことになる」 is who makes the decision: the former refers to the speaker themselves, while the latter refers to someone or something outside the speaker, such as another person, an organization, a rule, or circumstances. If you remember the contrast 「する = actively doing something yourself」 and 「なる = naturally becoming a certain way」, you can distinguish them correctly in most situations.
| Comparison | ことにする | ことになる |
| Decision-maker | The speaker | Another person, organization, rule, or circumstances |
| Core nuance | Subjective intention, active choice | Objective result, passive acceptance |
| Nature of the verb | する (actively doing) | なる (changing, becoming) |
| Meaning in the ている-form | A habit you decided on yourself | A rule, convention, or established arrangement |
| Common mistake | ❌ 出張することにした | ❌ Using it when you actually want to emphasize personal intention |
| Correct example | ✅ ダイエットすることにした | ✅ 出張することになった |
▌Comparison point 1｜Deciding for yourself vs being assigned by your company
❌ Unnatural: （when your company sends you on a business trip）一週間アメリカに出張することにした。
（The business trip was decided by the company, so using 「ことにした」 makes it sound as though you personally insisted on going on the trip.）
✅ Correct: 一週間アメリカに出張することになった。
Using 「ことになった」 fits the actual situation: “This was the company’s decision, and I’m simply being sent.” In terms of nuance, it places the decision-making authority back with the organization.
▌Comparison point 2｜A self-imposed habit vs a social rule
❌ Unnatural: 友達に会ったら、挨拶することにしている。
（Greeting friends when you see them is a general social norm, not a habit that you personally and deliberately decided on, so 「ことにしている」 sounds odd here.）
✅ Correct: 健康のために、毎日野菜を食べることにしている。
“Eating vegetables every day for health” is a routine the speaker has intentionally chosen for themselves, so this is natural. When talking about social rules such as traffic regulations, however, you would use 「ことになっている」 instead.
Why do Japanese speakers use 「ことになる」 even for things they decided themselves? Understanding the nuance through indirectness
Even when they have actually made the decision themselves, Japanese speakers often deliberately say 「ことになりました」. This is a type of indirect expression in Japanese that considers how the statement will sound to the listener. By presenting a personal decision as though it were a result that naturally came about, the speaker can soften their personal assertion and sound modest and tactful. This phenomenon clearly shows how closely Japanese linguistic nuance is connected to interpersonal culture in Japan, and it is also one of the most difficult points for native Chinese speakers to grasp intuitively.
Japanese society often places importance on not stating things too forcefully and not drawing excessive attention to the individual. If someone simply says 「私が決めました（I decided it）」, it can sound somewhat forceful or self-centered in certain situations. By switching to 「ことになりました」, the focus shifts away from “me” and toward “the outcome of events,” almost as if to say, “Thanks to everyone involved, this is how things have developed.” This way of placing oneself behind the outcome is part of the sense of interpersonal distance and consideration found in Japanese communication. To understand this way of thinking more deeply, you can also look at Japanese nuance distinctions.
▌Situation 1｜Announcing an engagement or marriage to family and friends
A：実は私たち、来年結婚することになりました。
じつはわたしたち、らいねんけっこんすることになりました。
Actually, we’re getting married next year.
B：おめでとうございます！
おめでとうございます！
Congratulations!
📌 Grammar Point: Getting married is clearly something the two people decided themselves, yet 「ことになりました」 is used. This does not mean that someone forced them to marry. Rather, it is a modest and gentle way to announce the good news, carrying a sense of “with everyone’s support, this is how things have worked out.” It sounds much softer than 「結婚することにしました」.
▌Situation 2｜Informing coworkers about a job transfer
A：4月から大阪支社で働くことになりました。
しがつからおおさかししゃではたらくことになりました。
I’ll be working at the Osaka branch starting in April.
B：そうなんですね。頑張ってください。
そうなんですね。がんばってください。
I see. Good luck.
📌 Grammar Point: Here, 「ことになりました」 clearly indicates that this is a personnel decision made by the company, with the speaker being the person transferred. Even if the speaker had personally expressed interest in the transfer, using this pattern allows the final decision to be attributed to the organization. It is a safe and appropriate choice in workplace communication.
Cultural Notes:
- Indirect expression in Japanese is not limited to 「ことになる」. It also appears widely in honorific language, passive constructions, and various vague sentence endings. The underlying principle is to avoid stating things too absolutely and to leave room for the other person. Understanding this makes it easier to see why Japanese communication can often sound indirect.
- The indirect use of 「ことになる」 also has an unspoken tendency: the more positive the event is and the more consideration needs to be given to how others perceive it—such as marriage, a promotion, or opening a business—the more likely speakers are to use this pattern to announce it modestly. By contrast, for a purely personal small decision such as what to eat today, simply using 「ことにする」 is sufficient.
- For native Chinese speakers, one of the best ways to practice is to pay particular attention when watching dramas to which pattern characters use when announcing important news. Over time, this helps build an intuitive sense for the nuance of “something I decided myself but present as an objective outcome,” which is more effective than simply memorizing rules.
How should Japanese 「ことにする」 be understood alongside Korean 「기로 하다」?
Anyone who has studied Korean will notice that Japanese 「ことにする」 for subjective decisions and 「ことになる」 for objective outcomes have very neat corresponding patterns in Korean. Understanding one side often makes the other easier to grasp.
◆ Similar grammar: -기로 하다（decide to ~）／-게 되다（end up ~ / come to ~）
In Korean, a verb stem followed by 「-기로 하다」 expresses one’s own decision and corresponds to Japanese 「ことにする」. Meanwhile, 「-게 되다」 expresses a result caused by outside forces or circumstances and corresponds to Japanese 「ことになる」. The two languages follow almost the same logic in distinguishing “active decision vs passive outcome.”
- Example: 내일부터 운동하기로 했어요.
Romanization: nae-il-bu-teo un-dong-ha-gi-ro hae-sseo-yo.
Translation: I decided to start exercising tomorrow.
「-기로 했어요」 expresses a decision made by the speaker and closely matches the subjective nuance of Japanese 「運動することにしました」. - Example: 회사 사정으로 부산에서 일하게 됐어요.
Romanization: hoe-sa sa-jeong-eu-ro bu-san-e-seo il-ha-ge dwae-sseo-yo.
Translation: Because of circumstances at the company, I ended up working in Busan.
「-게 됐어요」 indicates a result caused by circumstances at the company and corresponds to the externally determined nuance of Japanese 「大阪で働くことになりました」.
◆ Core difference from 「ことにする」:
Both can express a subjective decision, so their nuances overlap to a large extent. However, Korean 「-기로 하다」 is also very commonly used when describing an arrangement or agreement with another person（for example: 친구와 만나기로 했어요＝I arranged to meet a friend）. This sense of “agreement” is more explicit than with Japanese 「ことにする」 and is a subtle difference worth keeping in mind.
Distinguishing 「ことにする」 from 「ことになる」 is ultimately about understanding two different ways of viewing events in Japanese: “actively exerting your own will” and “going along with how circumstances develop.” One keeps the power of choice in your own hands, while the other presents the decision as something that comes from the surrounding environment or situation. In everyday communication, Japanese speakers often deliberately hide their own active intention out of consideration for the people around them, presenting events as though they naturally developed that way. That is precisely why this pair of grammar patterns needs to be revisited and absorbed through repeated exposure.
FAQs
The difference lies in who makes the decision. 「ことにする」 expresses a subjective decision made by the speaker, while 「ことになる」 expresses an objective result caused by another person, an organization, a rule, or circumstances. The key distinction is between the final verbs 「する（actively doing）」 and 「なる（changing or becoming）」.
This is an indirect form of expression in Japanese intended to sound modest and tactful. Presenting one’s own decision as “a result that naturally came about” softens personal assertion and allows the credit or responsibility to be shifted toward the surrounding people or organization. This kind of interpersonal restraint is especially common when announcing events such as marriage or a promotion.
「ことにしている」 refers to a personal habit that you decided on yourself and continue to follow, such as getting up at 5 every morning. 「ことになっている」 refers to established rules, laws, conventions, or arrangements, such as the rule that vehicles drive on the left. The former is self-imposed; the latter comes from outside.
A common mistake is using 「ことにする」 for something arranged by a company or another person. For example, if a business trip was decided by the company but you say 「出張することにした」, it sounds as though you personally chose or requested the trip. In that situation, 「出張することになった」 should be used instead.
The easiest method is to remember the contrast 「する = I actively do it」 and 「なる = it naturally becomes that way」. When deciding which pattern to use, ask yourself, “Did I make this decision, or was it caused by another person or by circumstances?” Paying attention to which pattern characters use when announcing news in Japanese dramas can also help build an intuitive feel for the distinction.