Learn the differences between ことにする (decide to) and ことになる (it has been decided) in Japanese.
•8 min read
Reviewed by GyanMirai Editorial Team•Last reviewed 2025-02-20
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Grammar Comparison JLPT N4 JLPT N4
ことにするvsことになる
ことにする vs ことになる: Japanese Decision Expressions Guide
Master ことにする (decide to) and ことになる (it has been decided) in Japanese. Learn the key differences with clear examples and practice exercises.
Read time: 7 min readSearches: 650+ monthlyUpdated: February 18, 2026
Understanding the difference between ことにする (koto ni suru) andことになる (koto ni naru) is one of the most important distinctions in Japanese grammar. This comprehensive guide will help you master when and how to use each one correctly.
Grammar Point A
ことにする (koto ni suru)
Decide to / Make it a rule to
Expresses a decision made by the speaker or subject. Active choice. Also used for personal rules/habits you've established.
Mistake #1Using ことにする for company/external decisions
Correction: Use ことになる when you're not the decision maker
If the company, boss, or circumstances decided — not you — use ことになる.
Wrong: 会社を辞めることにしました。(when fired) -- I decided to quit (but you were fired)
Correct: 会社を辞めることになりました。 -- It's been decided that I'll leave (external decision)
Mistake #2Using ことになっている for personal habits
Correction: Use ことにしている for rules you've set for yourself
ことになっている is for external/established rules. Personal habits use ことにしている.
Wrong: 毎日ジョギングすることになっている。(personal habit) -- There's a rule that I jog (sounds like someone else made the rule)
Correct: 毎日ジョギングすることにしている。 -- I make it a rule to jog daily.
Mistake #3Confusing ことになる with なる alone
Correction: ことになる = decision/outcome; なる = become
ことになる specifically expresses decided outcomes. Plain なる is just 'become.'
Wrong: 東京に転勤になった。(grammatically OK but less clear) -- Became a Tokyo transfer (less clear about decision)
Correct: 東京に転勤することになった。 -- It's been decided I'll transfer to Tokyo.
Mistake #4Using ことにした for things out of your control
Correction: Use ことになった for circumstances beyond your choice
If you didn't choose it, don't use ことにした. It implies you made the decision.
Wrong: 病気で入院することにした。(implies you chose to be sick) -- I decided to be hospitalized (strange agency)
Correct: 病気で入院することになった。 -- Due to illness, I ended up hospitalized.
Memory Tips
Tip 1
する = Active, なる = Passive
する verbs = you DO it (active). なる verbs = it BECOMES/happens (passive). Apply to ことにする/なる.
Example: 決める→ことにする (I decide) vs 決まる→ことになる (it gets decided)
Tip 2
Who Made the Call?
Ask: 'Did I decide, or was it decided?' I decided → ことにする. It was decided → ことになる.
Example: My diet choice → ことにした. Company transferred me → ことになった
Tip 3
〜ている Pairs
ことにしている = my personal rule. ことになっている = the established rule/custom here.
Example: 朝食べることにしている (my habit) vs ここでは脱ぐことになっている (the rule here)
Practice Exercises
Test your understanding with these practice questions. Click on your answer to see if you are correct.
0 / 6 answered
Q1
来週から禁煙する___。（自分で決めた）
I decided to quit smoking starting next week.
Q2
来月、海外出張に行く___。（会社の指示）
It's been decided that I'll go on a business trip overseas next month.
Q3
毎朝ジョギングする___いる。（自分のルール）
I make it a rule to jog every morning.
Q4
この学校では制服を着る___いる。
At this school, there's a rule to wear uniforms.
Q5
甘いものを控える___。健康のために。
I decided to cut back on sweets. For my health.
Q6
彼との結婚は取り消しに___。
The marriage with him has been called off.
Frequently Asked Questions
Technically yes, but it shifts responsibility. 辞めることになった suggests 'it ended up that I'll quit' (circumstances led to it) rather than 'I chose to quit.' Japanese speakers use this to soften announcements or avoid seeming overly willful.
ことになっている = established rule/custom (external). ことにしている = personal rule I've set. この会社では残業しないことになっている (company rule). 私は残業しないことにしている (my personal policy).
ことにする = decide to (one-time or policy decision). ようにする = try to / make sure to (ongoing effort). 早く寝ることにした (decided to sleep early). 早く寝るようにしている (trying to sleep early / making effort to).
ことになりました is commonly used in formal announcements because it sounds less personal and more objective. Companies often say 新製品を発売することになりました rather than することにしました (sounds more humble/official).
Yes! ことになる often describes decided future plans: 来月結婚することになっている (I'm getting married next month — it's decided). It's common for confirmed plans.
Summary
Use ことにする when...
Expresses a decision made by the speaker or subject. Active choice. Also used for personal rules/habits you've established.
Use ことになる when...
Expresses a decision made by circumstances, others, or external factors — not by the speaker's will. Passive/resultative.
Pro Tip: The best way to master the difference is through reading and listening practice. Pay attention to how native speakers use these grammar points in context.
