A scanning optical system includes a polygonal mirror that deflects the even number of beams, which are incident thereon along optical paths arranged symmetrically with respect to a plane perpendicular to a rotation axis thereof. An imaging optical system for converging the beams includes lenses for converging the beam, in an auxiliary scanning direction, respectively. The lenses have the same shapes, and are arranged symmetrically with respect to the plane. The orientation of the lenses on one side of the plane is oriented oppositely, in the auxiliary scanning direction, to the other lenses located on the other side of the plane.