In a similar fashion to に違（ちが）いない, に相違（そうい）ない is a structure that firmly states that something is 'without a doubt (A)', or that there is 'no mistaking (A)'. It may be used to strengthen one's own opinions, or to strongly agree with the opinion of another. Although 相違（そうい） may be used by itself meaning 'to differ', or 'to have a discrepancy', 相違（そうい）ない is considered as a set い-Adjective. When combined with ない, the literal meaning is just 'there is no discrepancy in (A)'.
に相違（そうい）ない may be used with verbs, い-Adjectives, な-Adjectives and nouns.

• 彼（かれ）は小学生（しょうがくせい）の頃（ころ）から一生懸命（いっしょうけんめい）勉強（べんきょう）をし続（つづ）けたので、いい大学（だいがく）に行（い）けるに相違（そうい）ない。
  Since he's been studying hard since he was in elementary school, there's no doubt that he can go to a good university.
• このコースはあの世界的（せかいてき）に有名（ゆうめい）なシェフによって作（つ）られたものなので美味（おい）しいに相違（そうい）ない。
  This course has been prepared by a world famous chef, so there is no doubt that it will be delicious.
• この街（まち）には電車（でんしゃ）が3分（ぷん）に１本（ぽん）来（く）るので便利（べんり）に相違（そうい）ない。
  In this city, a train comes every 3 minutes, so there is no doubt that it is convenient.
• あの事件（じけん）の犯人（はんにん）はあの人（ひと）に相違（そうい）ない。
  There is no doubt that that person is the culprit.

に相違ない will often follow から, highlighting that there is no mistaking the 'cause' of (A).

• 彼（かれ）がキョロキョロしているのは、嘘（うそ）をついているからに相違（そうい）ない。
  There is no doubt that he's being restless because he's lying.
• この技（わざ）が全国（ぜんこく）の大会（たいかい）で禁止（きんし）されているのは危（あぶ）ないからに相違（そうい）ない。
  There's no doubt that the reason why this move is forbidden in all competitions across the nation is because it's dangerous.