PURPOSE: To transmit data without adding a preamble signal to transmission data by permitting all stations to mutually transmit the data with the same timing reference.

CONSTITUTION: Clock signals generated from an oscillator 7 are converted into the optical signals of a wavelength band λ2 in a light transmitter 8 and are inputted to all optical multiplexer/demultiplexers 10 through a star coupler 1. The optical multiplexer/demultiplexers 10 output the optical signals of the wavelength λ2 to light receivers 9. Respective light receivers 9 reproduce the original clock signals from the received optical signals of the wavelength band λ2 and output them to the stations 2. Since respective stations 2 use the clock signals supplied form the light receivers 9, they operate in accordance with the single timing reference generated from the oscillator 7. Consequently, the data can be transmitted without adding the preamble signal to the transmission data.