Practice JLPTHow to tell them apart
に決まっている expresses a confident judgment about what is naturally expected or logically bound to happen, often based on common sense or obvious reasoning, while に違いない is a strong inference or guess about something the speaker cannot directly confirm, like deducing someone's nationality from appearance. Swapping them feels off: 安く買いたいに違っている sounds like you are guessing about a hidden fact rather than stating an obvious truth, and あの人は日本人に決まっている implies it is a foregone conclusion rather than a deduction.
- に決まっている Use に決まっている when stating what is obvious or logically bound to be true by common sense.
- に違いない・にちがいない Use に違いない when you are making a strong inference or guess about something you cannot directly confirm.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| に決まっている ni matteiru | Surely / bound to be | N3 |
| に違いない・にちがいない ni inai nichigainai | Must be / no doubt about it | N4 |
に決まっている ni matteiru
Surely / bound to be
Surely; be bound to; be a given; of course; must be; we all know that | A phrase indicating the speaker’s/writer’s strong conviction or motivation.
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お客さまだって安く買いたいに決まっているじゃないか。
Of course customers want to buy things cheaply.
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すぐ角のタバコ屋に行く程度の用事なら、車より自転車の方が便利に決まっている。
If you're just going on an errand to the tobacco shop around the comer, it's more convenient to go there by bike than by car — no question about it!
に違いない・にちがいない ni inai nichigainai
Must be / no doubt about it
There is no doubt that ~; must be ~; no doubt | The speaker is convinced that there is no mistake on his part in guessing something.
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あの人は日本人に違いない・違いありません。
That person must be Japanese.
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あの先生の試験は難しいに違いない・違いません。
That teacher's exams must be hard.
に決まっている and に違いない・にちがいない are JLPT N4–N3 grammar points that Japanese learners often mix up. This page compares them side by side — meanings, usage and example sentences — so you can tell them apart.