Two mirrors are placed in such a manner as to line in a Z-axis direction in a beam irradiation head. These mirrors are located at a position beyond a normal scan range of the laser light in the Z-axis direction and within a limit range of scanning of the laser light, and the angles of inclination of mirrors are adjusted so as to reflect incident laser light toward a PD. In the case where a lens is driven in a Z-axis direction (a lateral scanning direction), driving current values Ia, Ib of an actuator at the times when reflected light from these two mirrors enters the PD at the maximum light quantity are detected. A driving current Imz required to position a lens for irradiation at a neutral position and a drive amount .DELTA.zd of the lens for irradiation per a unit driving current are then determined based on the detected driving current Ia and Ib.