To provide a helmet to which theft-preventing function is added.

The helmet has a helmet body 2, a lock member 3 in which one end is attached to the helmet body 2 and either one engaging part 6 is formed at the other end, other lock member 4 attached to the helmet body 2 or formed in the helmet body 2 and equipped with other engaging part 9 with which the either one engaging part 6 is engaged and lock members 6a, 6b and 9a attached to the either one engaging part 6 and/or other engaging part 9 and retaining a state in which either one engaging part 6 is engaged with other engaging part 9, and a space closed by the helmet body 2, either one lock member 3 and other lock member 4 is formed by engagement of either one engaging part 6 with other engaging part 9 and the space is retained by the lock means 6a, 6b and 9a.