What it means
に違いない expresses a strong conclusion that something is almost certainly true. The speaker bases this judgment on evidence, knowledge, or circumstances, so it is stronger than a simple guess but does not usually claim absolute logical proof. It can follow a verb in plain form, an i-adjective, a noun, or the stem of a na-adjective. The polite counterpart is に違いありません.
Usage note
に違いない is an assertive expression and can sound more confident than でしょう. Do not attach it directly to a masu-form verb; use the plain form before it. With nouns and na-adjectives, use the noun or adjective stem directly, without だ.
How to use it
彼は医者に違いありません。
He must be a doctor.
この計画は安全に違いありません。
This plan must be safe.
彼の説明は正しいに違いありません。
His explanation must be correct.
田中さんは明日来るに違いありません。
Tanaka must come tomorrow.
彼は医者に違いない。
He must be a doctor.
この計画は安全に違いない。
This plan must be safe.
彼の説明は正しいに違いない。
His explanation must be correct.
田中さんは明日来るに違いない。
Tanaka must come tomorrow.