A multi-beam light scanning optical system includes n (n2) light source means having m (m2) light emitting points; beam synthesizing means for synthesizing nm light beams emitted by the light source means; deflecting means for deflecting nm light beams from the beam synthesizing means; an imaging optical system for imaging the nm light beams from the deflecting means on a surface to be scanned, wherein the surface to be scanned is scanned in a main scan direction with nm light beams by a deflection scanning operation of the deflecting means; an aperture stop provided between the beam synthesizing means and the deflecting means, wherein principal rays of the nm light beams are intersected at a position of the aperture stop, and the light beams which are adjacent to each other in a sub-scan direction on the surface to be scanned are the beams emitted from different ones of the light source means.