An optical scanning apparatus is constructed such that a light beam from a light source is formed into a linear image extending in a main scanning direction and is caused to be deflected at a constant velocity by virtue of a light deflecting system having a deflective reflecting surface which is located in a vicinity of an image formation position of the linear image. The deflected light beam is allowed to transmit through a scanning image formation lens so as to be converged into a beam spot on a surface to be scanned, thus causing the beam spot to scan the surface to be scanned at a constant speed. The scanning image formation lens is formed by more than two lens elements and has at least one special surface special surface in which a sub-scanning curvature changes in a main scanning direction according to a distance from an optical axis toward a periphery of the lens surface such that a line passing through centers of curvature in the sub scanning curvature is curved, and at least one of the special surfaces is formed so that the change of a sub-scanning curvature is non-symmetrical in the main scanning direction and the curvature has at least two or more than two extreme values.