A polygon mirror made of plastic is provided. The polygon mirror has a plurality of reflecting surfaces, a first surface intersecting the plurality of reflecting surfaces at a first side, a second surface intersecting the plurality of reflecting surfaces at a second side opposite to the first side, with a through hole provided to extend through the first surface and the second surface at a center of the polygon mirror. The polygon mirror includes a plurality of gate marks of injection molding. When viewed from an extending direction of the through hole, the gate marks are located on straight lines passing through the center and vertices of the first surface, and are rotationally symmetric with respect to the center.