A scanning optical system that emits a plurality of beams to a plurality of surfaces to be scanned, respectively. The scanning optical system includes a light source and a polygonal mirror. The plurality of beams incident on the polygonal mirror are inclined with respect each other in an auxiliary scanning direction. The scanning optical system further includes an imaging optical system that converges the deflected beams on the plurality of surfaces, respectively. The imaging optical system includes a front lens group and a plurality of rear lens groups. All the beams are incident on the front lens group, and then incident on the respective rear lens groups. Each of the plurality of rear lens groups has a shape which is designed in accordance with an angle of a beam incident thereon with respect to an optical axis of the front lens group.