PURPOSE: To make speed conversion and also the rise of a transmission clock frequency unnecessary, by generating and transmitting two series of optical signal system strings for an information system string to be transmitted.

CONSTITUTION: When the information system string 2 is inputted from a terminal 13 and a transmission clock 5 is inputted from a terminal 14 to a transmitter 12, outputs synchronized with the transmission clock 5 are sent from a gate circuit 16 at a part where a signal is 1 and from a gate circuit 17 at the part where the signal is 0 to light emitting elements 18 and 19. The light emitting elements 18 and 19 send light having wavelength λ1 and λ2 to an optical fiber 20 sensing to the outputs of the gate circuits 16 and 17, respectively. Meanwhile, the light receiving elements 22 and 23 of a receiver 21 sense to the light having wavelength λ1 and λ2, and send their emitting outputs to reproduction circuits 24 and 25. And at the reproduction circuits 24 and 25, respective reception code system string is reproduced, and those are added on the outputs at a timing generation circuit 26, then outputted from a terminal 30.