PURPOSE: To detect the generation of a fault of a transmission line, etc. before the generation of a transmission error by applying the input signal to two identification circuits of different identifying levels and detecting the discordance between outputs of both identification circuits.

CONSTITUTION: The optical signal supplied from a terminal 55 is converted into the electric signal by a photodiode 2. This electric signal is amplified by a preamplifier circuit 3 and an equalizing amplifier. The output of this amplification is supplied to a clock reproducing circuit 5 as well as the 1st and 2nd identification circuits 10 and 20 respectively. The voltage which minimizes an identification error is applied to a comparison terminal 15 of a comparator CMP11. While the voltage of a higher or lower identifying level than that of the voltage applied to the terminal 15 is applied to a comparison voltage terminal 25 of a comparator CMP21 of the circuit 20. The identification outputs of both circuits 10 and 20 are supplied to an exclusive OR circuit 6. Then the action margin of the reception level is tested by monitoring the detection signal sent from an output terminal 54 of the circuit 6 even in case the reception data of a reception data terminal 52 has no error.