に相違ない — Beyond All Doubt

この筆跡から見て、犯人は彼に相違ない。

Kono hisseki kara mite, hannin wa kare ni soui nai.

Judging from this handwriting, the culprit must certainly be him.

Noun 彼 + に相違ない expresses firm conviction based on evidence

この事故は彼の不注意が原因に相違ない。

Kono jiko wa kare no fuchuui ga gen'in ni soui nai.

There is no doubt that his carelessness was the cause of this accident.

Noun 原因 + に相違ない asserts firm conviction about the cause

彼女が優秀な人物であることは事実に相違ない。

Kanojo ga yuushuu na jinbutsu de aru koto wa jijitsu ni soui nai.

That she is an excellent person is doubtless a fact.

Noun 事実 + に相違ない asserts certainty in a formal register