PURPOSE: To simplify the circuit and to make a transmission path fully optical by using one of an optical signal with a different wavelength for transmission of a data string and using the other for a frame synchronizing signal, using a reception section to delay the latter so as to extract data from the data string.

CONSTITUTION: One of optical signals of two different wavelengths is used for transmission of a data string and the other is used for a frame synchronizing signal representing a head of the data string and they are sent from a transmission section. The reception section consists of an optical demultiplexer 11 demultiplexing two sent lights with two wavelengths, a delay circuit 12 and a switch circuit 13, a frame synchronizing signal is delayed for a prescribed time by the delay circuit 12 to read an object data bit and the delayed synchronizing signal is used for a switching signal of the switch circuit 13. When the switching signal is inputted to the switch circuit 13, the data bit is read from the data string and outputted. Thus, a clock recovery section and a frame synchronizing signal recovery section are not required. Furthermore, the transmission path is easily made fully optical by employing optical switches entirely.