the present invention provides a device for forming an image on a photoconductive image carrier in an image forming apparatus . the device comprises a light source for generating a light beam , an aperture for shaping the light beam generated by the light source , a deflector for deflecting the light beam shaped by the aperture , and an optical scanning imaging part for focusing the light beam deflected by the deflector onto the photoconductive image carrier as an optical spot and forming an image thereon . the deflector is provided with at least one blocking member for blocking flare beams that do not contribute to image formation . further blocking members may be provided to stationary parts of the device so as to block leaked light beams .