Disclosed is an optical scanning system and an image forming apparatus having the same, wherein, in one preferred form of the invention, the optical scanning system includes a light source device, a deflecting device for scanningly deflecting a light beam from the light source device, and an imaging optical system for imaging, upon a scan surface to be scanned, the light beam deflected by the deflecting device, wherein the imaging optical system consists of a single imaging optical element with a light exit surface having a concave shape with respect to a main-scan sectional plane, and wherein relations 0.5.ltoreq..phi.m/.phi.p.ltoreq.0.9 0