To provide a cap having a size adjustable without exchanging the covering member or changing the covering range.

A covering member 4 for sun shade, rain protection or cold protection use, and having a size to cover at least a part of at least the back of the head and the right and left temporal parts is attached to a crown part covering the top of the head in an unremovable or removable manner. A size adjusting part 5 for adjusting the inner diameter of the crown part is attached to the back of the crown part, and the covering member 4 is provided with an extension part 6 extensible and contractible by the size adjusting part 5 according to the change of the size of the crown part.