〜に違いない
Must be 〜に違いない
Meaning
A confident inference — "must be ~", "I am sure ~".
Formation
[plain form] + に違いない · [noun] / [na-adj] + に違いない (no だ)
Notes
Stronger than かもしれない and than はずだ — the speaker is convinced by evidence. A noun or na-adjective attaches bare: 犯人は彼に違いない, never ×彼だに違いない. に違いありません is the polite form.
Examples
電気（でんき）が消（き）えている。もう寝（ね）たに違（ちが）いない。
The lights are off. He must have gone to bed.
Breakdown
この字（じ）を書（か）いたのは彼（かれ）に違（ちが）いありません。
It must have been he who wrote this.
Breakdown
あの店（みせ）はいつも込（こ）んでいる。おいしいに違（ちが）いない。
That restaurant is always packed. It must be good.
Breakdown
彼女（かのじょ）は疲（つか）れているに違（ちが）いない。
She must be tired.
Breakdown
あんなに勉強（べんきょう）したのだから、合格（ごうかく）に違（ちが）いない。
He studied that hard, so he must have passed.
Breakdown
⚠️ Often confused with
〜そうだ (伝聞)
I heard that 〜そうだ (hearsay)
Hearsay — "I heard that ~ / they say that ~". Reports information obtained from another so...
〜そうだ (様態) / 〜そうに / 〜そうな
Looks like 〜そうだ (conjecture)
Visual conjecture based on appearance — "looks like ~ / seems about to ~". Based on what y...
〜らしい
Apparently / seems 〜らしい
Conjecture based on some evidence or hearsay — "apparently ~ / I hear that ~ / it seems ~"...
〜みたいだ / 〜みたいな / 〜みたいに
Looks like / seems 〜みたいだ
Conjecture based on appearance or impression — "it looks like ~ / it seems ~". More casual...
〜ようだ / 〜ような / 〜ように
Appears / seems 〜ようだ
Conjecture or resemblance — "it appears that ~ / it seems ~ / like ~". More formal than み...
〜かもしれない
Might / perhaps 〜かもしれない
Expresses possibility — "might / perhaps / may". Indicates something is possible but not c...
〜はずだ / 〜はずです
Should be 〜はずだ (expectation)
A strong expectation based on logic or evidence — "should be ~ / ought to ~ / is supposed...
〜はずがない / 〜はずありません
Cannot be / no way 〜はずがない
Strong denial of possibility — "there's no way ~ / it cannot be that ~ / impossible". The...
〜おそれがある
There is a risk that 〜おそれがある
"There is a risk / danger that ~".
〜かねない
Might well 〜かねない
"Might well ~", "could easily end up ~" — of a feared outcome.