An optical scanning device including a light source, a deflection device to deflect a light beam from the light source, an image focus optical system to focus the light beam deflected by the deflection device on a scanned surface to form an image thereon and scan a surface by the light beam deflected by deflection device to form an image thereon, a light path switching device provided between the light source and the deflection device, which switches a light path of the light beam emitted from the light source to deflect the light beam on different timings such that the light beam scans different surfaces.