PURPOSE: To prevent the saturation of a photodetector by comparing with a threshold signal obtained by an electric output signal corresponding to a part of an input light, and inhibiting that a high level pulse passes through the photodetector.

CONSTITUTION: As for an optical signal inputted to a beam splitter 10 from an optical fiber cable to be tested, a part thereof is sent to a first photodetector 12, the greater part of others is delayed 14, and thereafter, inputted to a high speed optical switch 16, and it is opened and closed by a control circuit 18. When this switch 16 is closed, its signal is converted to an electric signal by a second photodetector 20, amplified by a receiving amplifier 22, and thereafter, brought to sampling and processed. Subsequently, a part of an output of the amplifier 22 is inputted as a threshold electric signal to a comparator 24 through a threshold circuit 26, and compared with the signal from the detector 12. In this state, when the signal from the detector 12 exceeds the threshold signal, a control signal delayed 28 and synchronized is inputted to the circuit 18 and opens the switch 16, and a high level pulse is not supplied to the detector 20.