Scanning optical device and polygon mirror cover

摘要

A scanning optical device that includes a light source for emitting a light flux; a polygon mirror including a plurality of reflecting surfaces for deflecting the light flux to form a scanning beam spot on an image surface; a polygon mirror cover that covers the polygon mirror; and an optical sensor. The polygon mirror is provided with a mark on the top surface thereof that is eccentric from a rotation axis of the polygon mirror. Also, the polygon mirror cover includes a cylindrical rib formed on an inner surface of the top wall projecting toward the polygon mirror in a space between the top wall and the polygon mirror. The optical sensor is provided for detecting the mark and projects toward the polygon mirror from the inner surface of the top wall, through the cylindrical rib, such that the optical sensor forms a part of the cylindrical rib.