PURPOSE:To perform the maintenance work easily in a remote place by varying the level of a modulation signal of reference light to vary the gain of an optical receiver. CONSTITUTION:Reference light 3 becomes the modulation signal when it is made incident to a photodiode 1, and this signal is amplified by an amplifier 4 and passes through a BPF 8, and a peak value of this modulation signal is detected by a peak detector 9 and is inputted to a comparator 10. The peak value is compared with a reference voltage Vref, and a positive (negative) voltage is outputted if the peak value is smaller (larger) than the reference light voltage Vref. This positive or negative voltage is inputted to a voltage controller 11 to control the bias voltage of the photodiode 1, and a multiplication rate of photodiode 1 is changed. If the amplification degree of the amplifier 4 is changed by the positive or negative voltage, the amplification degree of the amplifier 4 can be controlled by the polarity of the output voltage of the comparator 10. Thus, the level of the modulation signal of the reference light 3 is changed to examine the characteristic of the optical receiver to the modulation signal of a main signal light 2 at this time.