に違いない|にちがいない

1. 1Expresses a strong conviction about a present state of affairs, such as someone's feelings or behavior: "must be."Ex. 1
2. 2Expresses a confident prediction that something will happen, often inferred from the current situation: "is sure to."Ex. 2

Common in both writing and speech, especially in narration and when making a confident guess about someone's state, motives or a likely outcome. The conviction may come from intuition or from observed evidence, as with a telling facial expression or heavy rain, but it is presented as the speaker's own firm judgment. It sounds much more certain than だろう or かもしれない.

• に相違ないN2「に相違ない」 has essentially the same meaning, but is stiffer and more formal, appearing mostly in academic papers, news reports, and formal speeches. After nouns and な-adjectives, you can also add 「である」 before it; in writing the two are interchangeable, but using it in casual chat sounds stiff.
• に決まっているN2「に決まっている」 emphasizes that something is self-evident based on common sense or the speaker's subjective conviction, and it has a strong colloquial flavor—it can sound overly assertive in formal settings; it can also stress that something goes without saying. When firmly concluding that a result is inevitable, the two are often interchangeable, but in writing or formal contexts 「に違いない」 is preferable.
• はずだN4「はずだ」 focuses on objective inference based on logic or known information, while 「に違いない」 leans toward the speaker's subjective certainty. Attach 「はずだ」 after nouns with 「の」 and after な-adjectives with 「な」. Forms like 「はずだった」 can express an expectation that fell through, which 「に違いない」 cannot.
• だろうN5「だろう」 also expresses conjecture, but only means 'probably...' with far less confidence, so replacing it noticeably weakens the tone. It can also be said with a rising intonation to seek the listener's agreement, a usage 「に違いない」 does not have. The polite form is 「でしょう」.
• かもしれないN4「かもしれない」 only indicates possibility without making any assertion, and its degree of certainty is far lower than 「に違いない」; in speech it can be shortened to 「かも」. Both attach to the same forms, except that after nouns and な-adjective stems no 「だ」 is added. They can be swapped, but the tone becomes uncertain.

に相違ない has the same meaning but is stiffer and more written, while に決まっている is more colloquial, emphatic and often emotional. はずだ presents an expectation reasoned from facts or knowledge in a more objective tone, and だろう and かもしれない express progressively weaker degrees of certainty.