A multi-beam scanning optical system includes a plurality of light sources, a deflecting unit which deflects a plurality of laser beams emitted from the light sources, and a scanning optical unit which focuses the laser beams deflected by the deflecting unit onto the surface of a photosensitive member. The scanning optical unit is set such that lateral chromatic aberration is overcorrected. In addition, among angles formed between each laser beam incident on the surface of the photosensitive member and the normal at the surface of the photosensitive member in a sub-scanning direction, the oscillation wavelength of the light source that emits a laser beam forming the minimum angle is set to a value smaller than the oscillation wavelength of the light source that emits a laser beam forming the maximum angle.