PURPOSE: To compensate drift based on the dark current or the like of a photodetecting element without generating deterioration of noise characteristics by adding a drift compensating signal to a reference voltage generator and finding out a difference between an output signal from the reference voltage generator and an output signal from a pre-amplifier.

CONSTITUTION: An optical signal inputted to the optical receiver is converted into an electrical signal by the photodetecting element 1 and properly amplified by the pre-amplifier 2 and the amplified signal is inputted to a differential amplifier circuit 3. On the other hand, an output signal from the reference voltage generating circuit 4 is inputted to the other input terminal of the circuit 3. The outputs 5, 6 of the circuit 3 are also inputted to the input terminals 8, 9 of an offset compensating circuit 7. The output of the circuit 7 is applied to the input terminal of the circuit 4 and negative feedback is applied so that the offset voltage is reduced when the offset voltage is high.