A scanning optical apparatus includes: a light source; a light deflector configured to deflect the light beam from the light source in a main scanning direction; an incident optical system disposed between the light source and the light deflector and configured to render the light beam from the light source nearly parallel in the main scanning direction and to converge the light beam in a sub-scanning direction to bring the light beam to a focus in proximity to the light deflector; and a scanning lens configured to focus the light beam deflected by the light deflector onto a target surface to form spot-like images. The incident optical system includes one or more lenses which provide at least one refracting surface and at least one diffraction surface, and φnS/φds