PURPOSE: To remove influences due to the dark current of a photodiode, by finding the difference signal between the conversion voltage based upon the output of the 3rd photodetecting surface opposed to the 1st photodetecting surface and that based upon the output of the 4th photodetecting surface opposed to the 2nd photodetecting surface and obtaining an optical system controlling signal from these two difference signals.

CONSTITUTION: A differential amplifier 17 receives a signal A0 from a buffer amplifier 34 at the plus input terminal and a signal C0 from a buffer amplifier 36 at the minus input terminal to amplify the difference (A0-C0) of both signals. In the same manner, a differential amplifier 18 amplifies the dfference (B0- D0) of both signals. An adder 19 adds the outputs of the differential amplifiers 17, 18 and the output of the adder 19 corresponds to F0=(A0-C0)+(B0-D0). Since the F0 can be modified by F0=(A0+B0)-(C0+D0), the output of the adder 19 is used as a focus controlling signal.