surely; without a doubt; used to assert that something is almost certainly true (often as 'ni chigainai').
彼は今頃もう着いているに違いない。
He must have arrived by now.
この店の料理は評判がいいから、美味しいに違いない。
This restaurant has a good reputation, so the food is bound to be good.
あの音は誰か歩いているに違いない。
That sound must be someone walking.