A scanning optical system deflects a light beam by a reflection surface of a polygon mirror toward an object surface through an image forming optical system so that the light beam scans on the object surface to form an image thereon. A light shielding member is disposed between the polygon mirror and the image forming optical system. The shielding member is an opaque plate disposed perpendicular to the optical axis of the image forming optical system. The light shielding member prevents a ghost light that is reflected by another reflection surface of the polygon mirror located adjacent to the reflection surface deflecting the light beam from entering the image forming optical system and impinging onto the object surface.