PURPOSE: To enable a plurality of communications at the same time and to reduce the probability causing the collision of communications at each station, by transmitting lights through the selection of light of a plurality of wave lengths by each station.

CONSTITUTION: When stations 1∼4 make transmission, a transmission logic section 10 selects the wave length not used for the communication on the circuit network among the photo trasmitters 11, 12, 13 and drive either one of them. The output of the transmitter thus selected is fed to an optical fiber 21 via the photo synthesizer 14 and photo distributor 15. On the other hand, the light fed from the fiber 21 is fed to photo receivers 17∼19 via the distributor 15 and brancher 16. Further, the section 20 can monitor the wave length used for communication in the circuit network, and in transmission, instruction is made to the section 10 so that the wave length not in used is selected. Thus, a plurality of communications can be made at the same time, and the probability for collision at each station can be reduced.