PURPOSE: To improve the S/N at a point of discrimination and to increase the photodetecting sensitivity, by discriminating the signal after getting the differential between the output signal given from a photodetector and the signal with the data train delayed by an amount equivalent to 1/2 time slot.

CONSTITUTION: The optical signal which is coded on the basis of a dipulse coding law is received at a photodetector 1 and then converted into an electric signal. This electric signal is amplified by an amplifying circuit 2 and converted into a signal A to be supplied to the plus terminal of a subtractor 4. At the same time, a part of the signal A is delayed by a delaying circuit 3 by an amount equivalent to 1/2 time slot and supplied to the minus terminal of the circuit 4 as a signal B. The output C of the circuit 4 is supplied to a discriminating circuit 8 via an equalizer 5. At the same time, a timing pulse E is supplied to the circuit 8 through a timing circuit 6. Thus a discriminating signal F is obtained.