PURPOSE: To improve resolution by easily forming a lens surface in an image formation optical system at low cost with high precision.

CONSTITUTION: The light beam scanning optical system is equipped with a deflecting means 2 which reflects and deflects the light beam 5 from a light source 11 to make a scan, and 1st and 2nd image formation optical systems 6 and 33 which are arranged before and behind the deflecting means 2 and images the light beam converged before the deflecting means on a scanned surface 41 through the deflecting means 2. As for the shape in the sub-scanning- directional plane in the 2nd image formation optical system 33, a radius K of curvature at a distance 2θ from the optical axis is represented, where R is a 1st radius of curvature on the optical axis, (r) is a 2nd radius of curvature coming into contact with the 1st radius R of curvature on the optical axis, and a straight line crossing the optical axis almost at right angles is a polar coordinate axis. Then the radius of curvature K at the angle 2θ from the optical axis is made to be given by K={R2+2Krω(2θ)+r}1/2.