に決まっている【にきまっている】

に決まっている expresses strong conviction or certainty about something, similar to "it must be the case that," "I'm sure that," or "certainly" in English. The speaker uses this pattern when they are confident about a conclusion, often based on common sense, personal experience, or strong belief. It carries a subjective tone – the speaker is asserting their own certainty rather than stating an objective fact.

Structure

This pattern attaches to the plain form of verbs, adjectives, and nouns:

• Verb (plain form)
  • に決まっている:
    かれ彼がくる来るにきまっている決まっている。

    He's definitely coming.
• い-adjective
  • に決まっている:
    そんなたかい高いみせ店はたかい高いにきまっている決まっている。

    A fancy place like that is obviously expensive.
• な-adjective (stem)
  • に決まっている:
    あのひとあの人はげんき元気にきまっているに決まっている。

    I'm sure that person is doing fine.
• Noun
  • に決まっている:
    はんにん犯人はかれ彼にきまっている決まっている。

    The culprit is definitely him.

Nuance

This expression conveys that the speaker considers the matter to be so obvious or certain that there is no room for doubt. It can sometimes sound dismissive or blunt, as if saying "obviously" or "of course." Because of this subjective and somewhat assertive tone, it is more common in casual conversation than in formal writing.

Polite form

In polite speech, にきまっています決まっています is used:

Related expressions

• にちがいない違いない also expresses certainty ("there is no doubt that…") but sounds more objective and is used in both spoken and written language. に決まっている is more colloquial and carries a stronger sense of personal conviction or even emotional emphasis.
• はずだ expresses expectation based on reasoning ("should be"), while に決まっている is a stronger, more emphatic assertion.