PURPOSE: To prevent the dispersion of a scan start position from occurring by installing a beam splitter capable of covering the prescribed angle range of a laser beam, separating an optical path to a light receiving plane from another optical path, and providing a photodetector on the other optical path.

CONSTITUTION: The laser beam 20 from a light source part 2 is reflected on the reflecting surface 51 of a rotary polygonal mirror 5. however, at this time, the irradiation point of the laser beam 20 on the reflecting surface 51 is moved from a corner part 52 on one side to a corner part 52 on the other side via the center part of the reflecting surface 51 according to the rotation of the rotary polygonal mirror 5, and also, the reflecting direction of reflected light is changed within the prescribed angle range. In such a case, the beam splitter 9 which separates the optical path to the light receiving plane 81 of a photosensitive drum 8 from another optical path is installed between an fθ lens 7 and the photosensitive drum 8. The beam splitter 9 is provided with length capable of covering the deflection angle of the laser beam 20. The power of the laser beam for a main scanning direction on a light receiving surface 81 can be uniformalized by covering the deflection angle.