PURPOSE: To facilitate stability control of a light source by concentrating light sources of terminal nodes.

CONSTITUTION: A central node CN generates a transmission wavelength light of each terminal node LN#k by a multi-wavelength light source OG, a multiplexer MUX1 multiplexes the lights and the result is fed to an optical fiber ring FR. On the other hand, the light from the FR is demultiplexed for each wavelength at a demultiplexer DEMUX, converted into an electric signal by a converter O/E and exchanged by an exchange switch SW for each destination, converted into an allocated wavelength light at the converter E/O, multiplexed by a multiplexer MUX2 and the result is sent to the FR. A demultiplexer DEMUX of a terminal node LN#k demultiplexes the transmission wavelength light and the reception wavelength light allocated to the node from the FR and a demultiplexer WDM demultiplexes the transmission wavelength light and the reception wavelength light, the optical receiver RX receives the reception wavelength light and an optical modulator MOD modulates the transmission wavelength light by a transmission signal and the multiplexer MUX multiplexes the modulation light with an undemultiplexed light and the result is fed to the FR.