判断・推量N2
〜に相違ない
undoubtedly... / certainly...
Structure
普通形/N + だ + に相違ない
Explanation
A very confident inference by the speaker: certainly, undoubtedly. More formal than に違いない. Attached to the plain form of verbs/adjectives or noun + だ.
Examples
あの笑顔を見れば、彼女は合格したに相違ない。
Judging by that smile, she must have passed.
これは間違いに相違ない。もう一度確認しよう。
This is certainly a mistake. Let's check again.
Note
Synonymous with に違いない but more formal, often used in reasoning and commentary.