An scanning optical system includes a light source having a plurality of light emitting points each of which emits a beam, a first optical system that collimates the beams emitted by the light emitting points in regard to a main scanning direction while converging the beams in an auxiliary scanning direction, a deflecting system that dynamically deflects the beams simultaneously in the main scanning direction, and a second optical system that converges the deflected beams on the scan target surface. Further, the second optical system includes a magnification adjustment element whose magnification mS in the main scanning direction on an optical axis thereof is approximately +1 and whose magnification mP in the auxiliary scanning direction on the optical axis thereof is approximately −1.