An optical scanning device includes a laser light source that emits a laser beam; a light deflector that deflects the laser beam; a scan-imaging optical system that focuses the laser beam deflected from the light deflector on a scanning surface to optically scan the scanning surface; a diffractive optical element receives the laser beam and diffracts a portion of the laser beam as a diffracted light; and a detecting unit that detects the diffracted light. The laser light source is a surface-emitting laser light source. Moreover, the diffractive optical element is made of plastic material having a linear expansion coefficient within a temperature variation range of the optical scanning device nearly equal to a rate of wavelength change of an emission wavelength of the laser light source within the temperature variation range.