PURPOSE: To remove unfavorable effects resulting from the dislocation and fluctuation of an optical axis by dividing a photodetector in equal parts with plural lines passing through the center of a photodetecting surface, and placing the photodetector on a jogging stand which is movable on a plane normal to the optical axis.

CONSTITUTION: The photodetecting surface of a photodetector 4 is divided in equal parts 4aW4d. The outputs of the small pieces 4aW4d are supplied respectively to comparators 14aW14d through each amplifier 7aW7d. Then a jogging stand 16, on which the photodetector 4 is placed, is controlled by a control circuit 17 so that the comparative outputs from each comparator 14aW14d may become equal. Thus the laser light spot is caught always at the central part of the photodetector 4. By forming in this way, the unfavorable effects resulting from the dislocation and fluctuation of an optical axis can be removed.