The Difference Between〜ことにする (koto Ni Suru) Vs 〜ことになる (koto Ni Naru)｜Japanese Grammar (N5–N3)

(Joy making a choice [Suru] vs. Joy accepting a rule [Naru])

“I decided to go.” (Iku koto ni shita)
“It has been decided that I go.” (Iku koto ni natta)
Both express a decision, but the “Controller” is different. 〜ことにする (Koto ni suru) is your active choice. 〜ことになる (Koto ni naru) is an outcome, rule, or external decision. Let’s master the difference between Willpower and Destiny with Joy and Ken!

🗣 Conversation Scene

Joy talks about her diet, and Ken talks about his work transfer.

Joy

私、今日からダイエットすることにしました！
毎日走ります！
(I have “decided to” go on a diet starting today! I will run every day!)

Ken

偉いですね。
実は僕、来月から東京へ転勤（てんきん）することになりました。
(That’s admirable. Actually, “it has been decided” that I will transfer to Tokyo next month.)

💡 Explanation

Did YOU choose? Or did it HAPPEN?

Point 1: 〜ことにする (Koto ni suru) = Active Decision

Focus: My Will / My Choice
“Suru” (to do) implies active volition. You looked at the options and chose one yourself.

タバコをやめることにした。
(I decided to quit smoking. [My willpower])
今日はピザを食べることにする。
(I decide to eat pizza today. [My choice])

Point 2: 〜ことになる (Koto ni naru) = Outcome / Rule

Focus: Result / External Force / Group Consensus
“Naru” (to become) implies the situation turned out that way. It is used for company orders, school rules, or when you want to sound humble (downplaying your own will).

転勤（てんきん）することになった。
(It was decided I will transfer. [Company order])
教室では日本語を話すことになっている。
(It is a rule/custom to speak Japanese in class.)

📊 Comparison Table

Phrase	Nuance	Situation
ことにする (Koto ni suru)	Active (Suru) “I decided”	Personal goals, Menu choices, New habits.
ことになる (Koto ni naru)	Passive (Naru) “It was decided”	Work orders, Rules, Destiny, Group plans.

🚧 Joy’s Mistake

(Joy tells Ken she is quitting her job)

❌ Wrong: 明日、会社を辞（や）める ことになりました。
(Meaning: “It has been decided that I will quit.” -> This sounds like you were fired or forced to quit!)

✅ Right: 明日、会社を辞める ことにしました。
(Meaning: “I decided to quit.” This shows it was your choice.)

🔥 Practice Quiz

Tap to check the answer!

Q1. (At a restaurant) I will have the curry.
(Karee o taberu ______.)

Answer: koto ni shimasu (ことにします)
You are making the choice right now.

Q2. (School Rule) We must wear uniforms.
(Seifuku o kiru ______.)

Answer: koto ni natte imasu (ことになっています)
“Natte iru” indicates an existing rule/custom.

Q3. (Reporting to parents) We decided to get married.
(Kekkon suru ______.)

Answer: koto ni narimashita (ことになりました)
Japanese people often use “Narimashita” for major life events to sound humble, implying it’s a blessing/destiny rather than just ego.

Keep practicing with JPLT!