A cap mounted to a mounting hole of a wheel, the mounting hole being formed to have a circumferential groove, includes: a circular body; a circular support; a circumferential projection that projects relative to an outer circumferential surface of the body in a radial direction; and a joint shaft. An outermost diameter of the circumferential projection is not more than an inner diameter of the mounting hole. The circumferential projection enters the circumferential groove of the mounting hole so that the cap is mounted to the wheel.