Provided is a scanning optical apparatus capable of forming a high quality image while manufacture of an imaging optical system is facilitated, a length of the apparatus in a direction of height is reduced, and unevenness of illuminance distribution (light amount distribution) on a scanned surface is reduced. The scanning optical apparatus includes a single deflecting unit which deflects respective light beams from multiple light source units toward multiple scanned surfaces by different deflection surfaces, multiple incident optical systems each disposed in each light source unit, multiple optical systems which image the respective light beams on the multiple scanned surfaces, and multiple optical path changing units disposed in the respective optical paths between the single deflecting unit and the multiple scanned surfaces. A thin-film characteristic of each of the optical path changing units is different between the respective optical paths.