PURPOSE: To obtain a focus error signal accurately and easily by making reflected light incident to three photodetectors arrayed in a bit direction.

CONSTITUTION: A photodetector 11 consists of three detectors 11aW11c, which are arrayed in a bit 7 direction. The differential output of the photodetectors 11a and 11b by the light spot 13 when an optical pickup and an optical disc are at an adequate spacing is a focus error signal. An information signal is obtained by the total sum of the three photodetectors 11aW11c. A tracking error signal is obtained by bisecting the photodetectors 11a and 11c by the straight line in the bit direction, respectively.