To prevent influence of the change of a driving current on the ratio between the light quantity of a laser beam passing a half mirror and the light quantity of a laser beam reflected by the half mirror.

An aperture 16 is provided between a half mirror 18 and a collimator lens 14 to shape a laser beam collimated by the collimator lens 14. For this reason, even when a driving current to drive a surface emitting laser 12 changes and the spreading angle and profile of the laser change, the light quantity intercepted by the aperture 16 becomes identical about a laser beam which passes the half mirror 18 and goes to a photoreceptor drum 28 and a laser beam which is reflected by the half mirror 18 and goes to a light receiving element 20. Accordingly, the light quantity ratio of the two laser beams does not change. Thus, the APC control of the light quantity of the surface emitting laser 12 can be performed precisely.