PURPOSE: To simplify and miniaturize the constitution of a photodetector, by forming a photodetection pattern with 6 radial photodetecting elements obtained by dividing a photodetecting element into two parts centering on a vertical axis and then dividing the element into two parts centering on the axes which are shifted from the vertical axis left and right by 45°, respectively.

CONSTITUTION: Photodetecting elements (a), (c), (d) and (f) having a 45° angle respectively among 6 photodetecting elements forming a photodetecting pattern detect a light in the direction (y); while the elements (b) and (e) having a 90° angle detect a light in the direction (x). Thus a focal error is detected. The detecting output of each photodetecting element is applied to adders Afy and Afx for detection of focal error or adders Atl and Atr for detection of tracking error. The outputs are equal to each other between the adders Afy and Afx when focusing is obtained. Thus a focal error signal delivered from a subtractor Sf is set at zero. When a photodetecting spot is on a track, the outputs are equal to each other between the adders Atl and Atr. Thus the tracking error signal delivered from a subtractor St is set at zero.