PURPOSE: To stably operate annular optical communication network, by supplying a minimum optical power required for an optical receiver to each optical receiver through distribution and giving the remaining optical power to the next station.

CONSTITUTION: When an optical transmitter 26 makes transmission, a controller 34 is operated so that all light can be transmitted to an annular optical fiber cable 21. At reception side, for example, a receiver 31 detects a received optical signal level, and the amount of distribution of a variable optical distributor 23 is controlled through a controller 35 so that the required minimum optical power can be received by the receiver 31. While the amount of distribution for variable optical distriburors 23∼25 is sequentially set, the transmitter 26 transmits meaningless data, the light obtained through circulation is received at an optical receiver and after the reception level is not in variance, a signal to be transmitted is transmitted.