A reflection type optical scanning system includes a cylindrical lens having positive power in the sub-scanning direction, a deflector, an image forming curved reflection mirror, and an anamorphic lens between the curved reflection mirror and an object surface to be scanned and having at least one toroidal surface with positive power in the sub-scanning direction, along a light path in this order from a light source side. The deflector is arranged so that light emitted from the cylindrical lens is made incident thereupon at a predetermined incident angle in the sub-scanning direction. The curved reflection mirror is made of a barrel-shaped toroidal surface, a cross-sectional in the main scanning direction being a concave arc having a single center of curvature or an arc and a cross-sectional shape in the sub-scanning direction being an arc having a single center of curvature, with respect to the incident light. The position of the center of curvature varies depending on the image height in the main scanning direction. The curved reflection mirror is inclined so that the light incident thereupon from the deflector is reflected by the curved reflection mirror in the sub-scanning direction at a predetermined angle toward the object surface. The axis of the anamorphic lens is deviated from the optical axis of the optical scanning system in the sub-scanning direction.