An optical scanning apparatus has a scanning lens with a rotationally asymmetric aspheric surface having individual curvature radii in the main scanning direction and in the sub-scanning direction. The curvature radius in the sub-scanning direction is asymmetrically increased in the lateral direction with the distance from the optical axis. A field curvature aberration which may be generated in the sub-scanning direction on the scanning surface due to an oblique incident of optical beams on the rotating polygonal mirror or the movement of the reflective surface caused by rotation of the rotating polygonal mirror is compensated for by the scanning lens and the focusing properties of the system can be kept at a level of high resolving power.