PURPOSE: To highly accurately detect a light beam with a simple constitution without necessitating an expensive scanning image forming optical system.

CONSTITUTION: Laser beams emitted from two laser light sources having the different wavelength are synthesized to one optical path by a synthesizing mirror, the synthesized light is deflected by a polygon mirror, and transmitted through the image forming optical system, then, the synthesized light is separated into two laser beams again by a separating mirror so that images may be formed on two different points on a photoreceptor. In this case, the synthesized light is partly guided to a beam detecting optical system by a beam splitting mirror installed on the way of a synthesized light path, the deviation of the image forming position owing to the different wavelength on a beam detecting slit 22 is corrected by using a grating lens 21, and then, the beam is highly accurately detected by a split sensor 23.