To suppress deterioration of transmission quality while reducing a manual load by preventing an inter-channel cross talk from deteriorating even if an output light wavelength of a light transmitter fluctuates.

A part of a light signal transmitted from a light transmitter is branched by a directional coupler 1 just before it is introduced into a light amplification part 2 and it is divided into two systems by a directional coupler 3. The light signal of one system is introduced to a Pin photodiode 4 as it is and is photoelectrically converted. The light signal of the other system is introduced into the Pin photodiode 6 through a light filter 5 and is photoelectrically converted. The photoelectrically converted outputs of the Pin photodiodes 4 and 6 are given to a differential amplifier 8 and the amplification output is compared with threshold voltage that a reference voltage source 11 gives by a comparator 10. When the output of the differential amplifier 8 exceeds the threshold, the light amplification part 2 is turned off by an output cut circuit 12.