〜かもしれない vs 〜に違いない: Degrees of Certainty in Japanese
The core difference
Both attach to a plain-form clause and both express a guess, but they differ on one axis — how confident the speaker is:
| Form | Core idea | Example |
|---|---|---|
| 〜かもしれない | 'might / maybe' — LOW certainty (~50% or less); an unsure guess | 雨が降るかもしれない = It might rain (I'm not sure) |
| 〜に違いない | 'must be / I'm sure' — HIGH certainty; a confident conclusion from evidence | 雨が降るに違いない = It must be going to rain (I'm certain) |
The rule of thumb is simple: かもしれない keeps the door open ("maybe, maybe not"), while に違いない closes it ("there's no other answer"). The feeling follows from the certainty — かもしれない hedges, に違いない commits.
See it in one situation
Same sky, same clouds, two very different levels of confidence:
午後は雨が降るかもしれない。
ごごは あめが ふる かもしれない。
It might rain this afternoon.
かもしれない — a tentative guess; it could just as easily stay dry.
空を見て、午後は雨が降るに違いない。
そらを みて、ごごは あめが ふる に ちがいない。
Looking at the sky, it must be going to rain this afternoon.
に違いない — the dark sky is evidence, so the speaker is confident.
The first is a shrug: it could rain. The second is a conclusion drawn from what ヤッタン can see — the sky has made up his mind for him.
〜かもしれない — the tentative "might / maybe"
Use かもしれない to float a possibility you're not sure about. It attaches to the plain form of verbs and い-adjectives, and to nouns/な-adjectives directly (drop な). In casual speech it often shortens to かも.
モチはもう寝ているかもしれない。
モチは もう ねている かもしれない。
Mochi might already be asleep.
A guess with no strong evidence — he could be awake just as easily.
この店は今日休みかもしれない。
このみせは きょう やすみ かもしれない。
This shop might be closed today.
Noun + かもしれない (no な/だ); still just an unsure possibility.
Because かもしれない is low-commitment, it's the polite, cautious choice when you genuinely don't know: 違うかもしれません ("I might be wrong") softens an opinion without overstating it.
Full details: 〜かもしれない guide.
〜に違いない — the confident "must be"
Use に違いない when the evidence points one way and you've reached a firm conclusion. It attaches to the plain form of verbs and い-adjectives, and to nouns/な-adjectives directly (drop だ/な). It's stronger and more written/emphatic than かもしれない.
電気がついている。ヤッタンは家にいるに違いない。
でんきが ついている。ヤッタンは いえに いる に ちがいない。
The lights are on. Yattan must be home.
The lights are the evidence, so the speaker is sure — not just guessing.
彼女の笑顔を見れば、合格したに違いない。
かのじょの えがおを みれば、ごうかく した に ちがいない。
From her smile, she must have passed.
に違いない draws a confident conclusion from a visible clue.
The nuance is "I'm certain — there's no other explanation." That's why it needs some basis: に違いない without any grounds sounds oddly overconfident, whereas かもしれない is fine on a hunch.
Full details: 〜に違いない guide.
A useful contrast: the certainty scale
- かもしれない ≈ 50% or less — "it might be." You'd accept being wrong.
- に違いない ≈ 90%+ — "it must be." You'd be surprised to be wrong.
- Between them sits だろう / でしょう ("probably"), a middle guess. If you're weighing options, ask whether you'd bet on it: no → かもしれない, yes → に違いない.
So the two are not interchangeable: swapping in に違いない turns a cautious "maybe" into a firm "definitely," which can sound too strong if you don't actually have the evidence.
Common mistakes
- Using に違いない for a pure hunch. With no evidence, "he might come" is 来るかもしれない, not 来るに違いない — the latter claims a certainty you don't have.
- Leaving だ before に違いない. For nouns, say 学生に違いない, not 学生だに違いない. Drop the だ (and the な on な-adjectives).
- Treating かもしれない as strong. かもしれない is a weak guess; if you're genuinely confident, it undersells you — use に違いない.
- Forcing English 'must' onto obligation. に違いない is inference ("must be true"), not duty. "You must go" (obligation) is a different grammar (〜なければならない), not に違いない.
Quick recap
- かもしれない = "might / maybe" → LOW certainty (~50% or less); an unsure guess.
- に違いない = "must be / I'm sure" → HIGH certainty; a confident conclusion from evidence.
- かもしれない is an unsure guess; に違いない is a confident verdict.
- Nouns/な-adjectives: drop だ/な before both (休みかもしれない, 学生に違いない).
- Scale: かもしれない < だろう/でしょう < に違いない.
Your turn
Choose かもしれない or に違いない for each sentence based on how much evidence and certainty it carries.
＿＿＿に入れるのに最もよいものを選びなさい。
彼が犯人＿＿＿。
Frequently asked questions
What's the quickest way to choose between かもしれない and に違いない?
Ask how sure you are. かもしれない is a low-confidence 'might / maybe' (~50% or less) that you'd accept being wrong about; に違いない is a high-confidence 'must be / I'm sure' drawn from evidence. Unsure guess → かもしれない, confident conclusion → に違いない.
Does に違いない mean 'must' like an obligation?
No. に違いない is inference — 'it must be true' based on evidence. The 'must' of obligation ('you must go') is a different grammar such as 〜なければならない. に違いない only expresses certainty about a fact.
How do they attach to nouns and な-adjectives?
Both attach directly, without だ or な. Say 休みかもしれない and 学生に違いない (not 休みだかもしれない or 学生だに違いない). With verbs and い-adjectives, use the plain form.
Where does だろう / でしょう fit in?
It's the middle of the scale — 'probably.' The order of confidence runs かもしれない (maybe) < だろう／でしょう (probably) < に違いない (must be). Pick に違いない only when you have real grounds to be certain.
The patterns compared here
Full guide for each pattern in this comparison: