PURPOSE: To prevent the dispersion of the scan start position of a laser beam by performing main scan and auxiliary scan at different positions by means of the laser beams emitted from different light source parts, respectively.

CONSTITUTION: The laser beam 20A from the light source part 2A is reflected on the reflecting surface 51 of a rotary polygonal mirror 5, and it becomes the laser beam 22A, and the laser beam 20B from the light source part 2B is reflected on the reflecting surface 51 of the rotary polygonal mirror 5, then, it becomes the laser beam 22B. At this time, the reflecting positions 21A. 21B on the reflecting surface 51 are moved from a corner part 52 on one side to the 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 directions of the laser beams 22A, 22B after reflection are changed within a prescribed range, respectively. In such a case, since the directions of the laser beams 22A, 22B reflected on the rotary polygonal mirror 5 are changed at constant angular velocity, such correction to obtain constant velocity on a light receiving surface is performed by an fθ lens 7.