PURPOSE: To compensate the fluctuation in the output from a light source and the transmission loss of light with simple constitution by performing electrical signal processing by the scoop characteristic of a semiconductor laser of a hetero junction type.

CONSTITUTION: A semiconductor laser of a hetero junction type has the scoop characteristic at which the internal resistance value decreases if there is reflected light. The voltage generated in a resistance 70 is increased by such scoop characteristic as compared to the case when there is the reflected light and there is no reflected light. The increased component associates with reflected light I and reflected light I2.α. Only the reflected light I2 is taken out by an operationl amplifier 80. On the other hand, the input light I5 of a photodetector 17 is proportional to the output light I1 and the reflected light I2.α and can be expressed by I5=k.α.I1sinωt, where k is a proportional constant. Therefore if the ratio between the input light I5 and the reflected light I2.α is taken by an arithmetic circuit 140, there is obtd. I5/α.I2=k.α.I1sinωt/α.I2=k.I1sinωt/ I2 and the transmission loss α of the 1st single mode optical fiber 150 is negated.