Respective first semiconductor lasers 44a and 44b and respective second semiconductor lasers 45a and 45b are arranged on a drive substrate 46 (YZ plane) such that the emission directions of light beams L1 to L4 of the respective first semiconductor lasers 44a and 44b and the respective second semiconductor lasers 45a and 45b are perpendicular to the YZ plane. On the YZ plane, the respective first semiconductor lasers 44a and 44b and the respective second semiconductor lasers 45a and 45b are divided to two mutually different lines y1 and y2 in the lateral direction perpendicular to the rotation axis of a polygonal mirror 42, and the respective first semiconductor lasers 44a and 44b and the respective second semiconductor lasers 45a and 45b are arranged on respective different lines z1 to z4 in the vertical direction as the rotation axis direction of the polygonal mirror 42.