Disclosed is an optical scanning system and an image forming apparatus, being small in size but able to produce high-quality image outputs, wherein the optical scanning system includes a deflecting device for scanningly deflect a light beam from a light source in a main scan direction, and an imaging optical system for imaging, upon a surface to be scanned, the light beam deflected by a deflecting surface of the deflecting device, wherein the deflecting surface is reciprocatingly moved to reciprocatingly scan the surface in the main scan direction with the light beam deflected by the deflecting surface, wherein a first direction refers to a direction of a phase difference of wavefront aberration in the main scan direction between a marginal ray and a principal ray of the light beam reflected by the deflecting surface at an effective deflection angle of the same corresponding to a largest scan position in an effective scan region, the phase difference being produced as a result of reflection of the light beam by the deflecting surface, while a second direction refers to a direction of the phase difference of wavefront aberration in the main scan direction between a marginal ray and a principal ray of the light beam reflected by the deflecting surface at an effective deflection angle thereof, the phase difference being produced as a result of transmission of the light beam through said imaging optical system, and wherein at least one optical system inside the imaging optical system is provided with at least one optical surface of non-arculate shape in a main scan section, so as to assure that the first and second directions are made opposite to each other.