To achieve a compact optical scanning device which is easily attached to a housing without needs for a large diameter to a second optical system focusing polarized light flux on a scanned surface.

Optical scanning is performed by coupling luminous fluxes from a light source part 10 through a first optical system 11a, 11b to guide the flux to a light deflection part and focusing the deflected luminous fluxes through an optical deflector on the surface to be scanned 14 in a second optical system 13 as a light spot. The reflected luminous flux to go to an optical scanning start position A is detected by a third optical detector 17 through an optical system 15, and a synchronizing signal for starting the optical scanning is generated. Conditions have been provided, under which the first through third optical systems are not mechanically interfered with each other and are able to be arranged compactly.