に違いない【にちがいない】

The expression に違いない (にちがいない) is used to express strong certainty or conviction – "there is no doubt that…," "I'm sure that…," or "it must be…." The speaker is stating their firm belief based on evidence, reasoning, or intuition.

Formation

に違いない attaches to the plain form of verbs, adjectives, and nouns:

• Verb (plain form)
  • に違いない:
    かれ彼はしっている知っているにちがいない違いない。

    He must know (about it).
• い-adjective
  • に違いない:
    あのみせ店はたかい高いにちがいない違いない。

    That shop must be expensive.
• な-adjective + な
  • に違いない:
    かのじょ彼女はげんき元気なにちがいない違いない。

    She must be doing well.
• Noun + に
  • 違いない:
    はんにん犯人はかれ彼にちがいない違いない。

    The culprit must be him.

With な-adjectives, な is required before に違いない, and with nouns, the pattern becomes Noun + に + 違いない (which looks the same but に serves as the copula).

Nuance

に違いない expresses a subjective but strong conviction. The speaker feels very confident in their judgment, though it is still ultimately their personal assessment rather than objective fact. It is stronger than かもしれない (might) or だろう (probably), and closer in strength to "must be" or "I'm certain that" in English.

Polite form

In polite speech, に違いない becomes に違いありません.