An optical scanning device includes at least one scanning unit having a deflector for scanningly deflecting a light beam from a light source, and an imaging optical system for imaging the light beam scanningly deflected by the deflector upon a plurality of photosensitive drums, wherein, at each of a plurality of light paths extending from the deflector to the plurality of photosensitive drums, at least one reflection member for turning the light path into a sub-scan direction is provided, wherein the plurality of light paths are different in the number of the reflection members, wherein a polarization direction of a light beam incident on each reflection of the plurality of light paths is S-polarized at an optical axis of the imaging optical system, wherein the reflection surfaces of all the reflection members of the plurality of light paths have the same film structure, and wherein the difference among the plurality of light paths of a total turn angle defined by the reflection surface or surfaces of the reflection member or members in the sub-scan direction at the optical axis of said imaging optical system, is not greater than 40 degrees.