PURPOSE: To prevent a change of characteristics within a data device which works in an AC connection, by synthesizing an optical signal obtained by converting a digital code and an optical signal of a certain pattern and then transmitting the synthesized signal.

CONSTITUTION: An electric signal sent from a central control device 1 is turned into an optical signal of wavelength λ1 by an optical transmitter 14 and then transmitted to an optical branching filter 11. An optical transmitter 23 produces an optical signal having a specified pattern of wavelength λ1. The both optical signals are synthesized by an optical branching device 9. This synthesized optical signal is set at about 50% duty ratio. At a terminal device, an optical signal of wavelength λ1 produced from an optical transmitter 24 and an optical signal which is transmitted are synthesied through an optical branching device 3. As a result, the original signal is separated and extracted to be supplied to an optical receiver 18 via an optical branching filter 12. The signal sent from a terminal device is sent to the device 1 through a backward route.