A rotating polygon mirror includes a polygon cylinder having a reflection mirror surface formed on at least one of the sides thereof, and mount portions formed on an upper surface and a lower surface thereof, which are to be held by mount members of a laser scan type optical system. At least one of the mount portions on the upper and lower surfaces has such a shape that, when two rotating polygon mirrors are stacked with their non-corresponding mount portions abutting each other, a reference surface formed in at least one of the mount portions of one rotating polygon mirror does not contact the mount portion of the other rotating polygon mirror.