A scanning optical system with a light beam deflected by an optical deflector is scanned on a scanning surface in a main scanning direction through the scanning optical system. The scanning optical system comprises an image forming lens system having a refracting power at least in a direction of the main scanning direction and an optical member having at least one deviated surface whose center of radius of paraxial curvature is deviated from a main scanning plane in a sub-scanning direction which is normal to the main scanning direction. The main scanning plane is defined by a plane that the light beam scans. A light interrupting member for interrupting a ghosting light reflected by external or internal surfaces including the deviated surface in the image forming lens system is provided. The light interrupting member is disposed between the image forming lens system and the scanning surface to prevent ghosting light from reaching to the scanning surface.