A headrest is mounted to a seat back by inserting a support into a support hole of a holder and inserting a stay of the headrest into to stay hole of the support inserted into the support hole of the holder. A gap occupying part provided in the support moves between an advanced position at which the gap occupying part protrudes from an outer wall of the support and is capable of touching an inner surface of the support hole, and a retracted position at which the gap occupying part is retracted to an inner side of the stay hole from the advanced position. The gap occupying part moves toward the retracted position from the advanced position when the support is inserted into the support hole, and moves toward the advanced position when the stay is inserted into the stay hole.