PURPOSE: To prevent the occurrence of such a case that data transmission becomes impossible by outputting optical signals for diagnosis having a wavelength which is different from that of optical signals for data transmission to the optical transmission line and diagnosing the deterioration of the optical transmission line based on the receiving level of the optical signals for diagnosis.

CONSTITUTION: At the time of data transmission from a data processor 1A to another data processor 1B, the processor 1A outputs transmission information to an optical transmitter 2A. Upon receiving the information, the transmission controlling section 4A of the transmitter 2A outputs transmitting optical signals having a wavelength n1 corresponding to the transmitting data to an optical demultiplexer/multiplexer 5A and at the same time, diagnostic optical signals having a wavelength n2 obtained by optically modulating diagnostic data constituted of a repetition of a prescribed bit pattern are outputted to the multiplexer 5A from a diagnostic signal transmitting section 6A. The multiplexer 5A multiplexes both optical signals and output the multiplexed optical signals to an optical fiber 7A. The optical signal outputted to the fiber 7A are branched into optical fibers 7B, 8A and 8B by means of an optical star coupler 11 and the received diagnostic optical signals ONa and ONb having the wavelength n2 are respectively inputted to diagnostic signal receiving sections 10A and 10B. The sections 10A and 10B check whether or not a specific bit pattern exists in the diagnostic data obtained by demodulating the optical signal ONa and ONb into electric signals and inform of the state of the transmission lines.