A polygon mirror includes a resin member, a first surface, a second surface facing the first surface, an inner surface joining the first and second surfaces to surround a through hole extending from the first to the second surface, and includes outer surfaces joining the first and second surfaces. A first segment between the first surface and the outer surfaces is defined as a first reference of a height in a direction from a second segment between the second surface and the outer surfaces to the first segment. The first surface includes a projection that forms the inner surface and projects from the first reference toward a side opposite to the second surface. The second segment is defined as a second reference of the height in the direction. The second surface includes a depressed portion distant from the outer surfaces and depressed from the second reference toward the first surface.