To provide a cap having air permeability and also excellent in operability of size adjusting.

This cap 2 includes a slide adjuster 8, wherein the slide adjuster includes a band 10 and a buckle 20 made from a synthetic resin, the engaging claw of the buckle is engaged with a saw tooth-like engaging groove formed at the outer surface of the band, the height of the compression part of the buckle for releasing the engagement is set to be the same as or lower than the height of the ceiling frame of the buckle, the buckle is fixed at a sweatband 4 through the ceiling frame so that the compression part faces the inside of the cap, the ceiling frame of the part fixed at the sweatband 4 is formed from a flat plate, and the band and/or the core material linked with the band is exposed to the inside (a surface making contact with a head part) of a size origin (the lowest part of a crown).