PURPOSE: To make an optical frequency of each channel stable to a different frequency by using an optical multiplexer having a specific transparent frequency, applying synchronization detection to a transmitted light intensity signal and modulation signal and feeding back the result to a light source negatively.

CONSTITUTION: Modulation signals 10,20,30,40 having four sets of independent information become optical signals 12,22,32,42 subject to frequency shift keying modulation by transmitters 11,21,31,41 of each channel. The four optical signals 12,22,32,42 become an optical frequency multiplex signal 51 by an optical multiplexer 50 comprising combination of Mach ender type interference filters and the signal is sent through a photocoupler 56. Part of the optical frequency multiplex signal 51 divided by the photocoupler 56 passes through a photodetector 52 and an amplifier 53 and is distributed to each of channel transmitters 11,21,31,41 as a feedback signal for frequency stabilization.