A steel door capable of sufficiently resisting even the invasion of a forcible and brutal burglar though its structure is simple and providing a high added value for crime prevention without impairing aesthetic appearances such as shape and design. The steel door (1) comprises an indoor panel (7) and an outdoor panel (6). A sound isolation core material (B) is filled in a space part (A) formed between the inner surfaces of these panels (6) and (7), and a highly hard metal plate (15) thicker in plate thickness than at least one panel among the panels (6) and (7) is tensely installed between the entire inner surface of the panel excluding the mounting part of door structural members and the sound isolation core material (B).
