PURPOSE: To eliminate an overhead due to a preamble by operating all stations synchronously with a distributed clock at all times so as to make a time required for clock recovery actually zero.

CONSTITUTION: A phase locked loop oscillator 10 is operated by a master clock supplied from a clock photodetector 9 and uses its output and a pseudo data in a wavelength λD is sent. The signal is made incident on its own data photodetector 11 via a star coupler 21 and an optical multiplexer and demultiplexer 8. A phase comparator 7 outputs an error signal to a phase shifter 6 to adjust the phase shifter so that a phase shift in an output of the phase locked loop oscillator 10 is within a prescribed value in terms of the error signal. Thus, the phase of a transmission data is synchronized with the master clock, the station exits from the startup mode, the phase shift of the phase shifter 6 is fixed and the monopoly right of a bus is left. The procedure above is repeated to each slave station (node) to take synchronization of the entire system.