Disclosed in an optical scanning apparatus which includes a deflecting unit comprised of a rotary polygon mirror for deflecting a light beam radiated from a light source unit, and a scanning optical system for guiding the light beam deflected by the deflecting unit to a surface to be scanned. In the optical scanning apparatus, individual elements are set such that a diameter of a circumscribed circle of the rotary polygon mirror, the number of deflecting facets of the rotary polygon mirror, an incident angle of the light beam on the deflecting facet at the time when the light beam scans a scanning center, a maximum swing angle of the deflecting facet at the time when an effective scanning range is scanned, and a magnification of the scanning optical system in a sub scanning section can satisfy a predetermined condition, thereby reducing an unevenness of pitches due to a deflecting-facet fall of the rotary polygon mirror, and readily achieving a highly precise and fine image.