PURPOSE: To provide the long-distance optical communication system which extends the transmittable distance of an optical signal with a relatively simple constitution and transmits the optical signal regardless of a fault like the stop of power supply to an active coupler.

CONSTITUTION: A part of an optical signal S1 transmitted toward an optical receiver is branched and outputted by an active coupler 10, and a synchronizing optical signal S3 having a second wavelength is transmitted toward the receiver synchronously with an optical signal having a first wavelength out of a branch optical signal S2. Further, a part of an optical signal S7 is branched and outputted by an active coupler 30, and a synchronizing optical signal S9 having the first wavelength is transmitted toward the optical receiver from the active coupler 30 synchronously with an optical signal having the second wavelength out of a branch optical signal S8. The output level of the optical signal transmitted from the active coupler 30 toward the optical receiver is raised, and long- distance optical communication is possible.