PURPOSE: To compensate the efficiency deterioration of a light emission element and make trouble detection possible by controlling the transmission power by a part of the output of the light emission element.

CONSTITUTION: Modulated signal 1 is applied to light emission element 3 through transmission amplifier 2, and optical output 4 is transmitted to fiber 5. Then, a part of optical output 4 is made incident onto photo detection element 12 through feedback optical fiber 11, and the output is caused to pass through receiving amplifier 13 to obtain power feedback signal 7. Then, the difference between this signal and power command signal 6 is defined as power control signal 8, and signal 8 is applied to amplifier 2. As a result, the output deterioration of the light emission element dependent upon elapse of years can be compensated and trouble detection is possible