To provide a helmet capable of readily and precisely attached to a head part, and having a light weight.

This helmet has plural air bags 34, 34... at the positions facing to a ring-like head band 24 along the inner periphery wall of a cap body 30. The air bag 34 is capable of being expanded and shrunk so that the air bag 34 may act a size-regulating function of the head band, and composed of a flexible synthetic resin such as a polyethylene, a polypropylene, a polyester, a polyamide and a vinyl chloride. The left half from the two-dot chain line shows the shrunk state of the air bags 34, and the right half of the two-dot chain line shows the expanded state of the air bags 34. The expansion and shrinkage of the air bags 34 are carried out by a small, light and manual air pump 36.