PURPOSE: To allow the reception side to identify whether a cause to interruption of transmission resides in the transmission side or an optical fiber by supervising always an output of an optical transmission panel of a repeater and transmitting an output interruption signal from the optical transmission panel of other system to the reception side when the output interruption is detected.

CONSTITUTION: When an optical output of the optical transmission panel OSm of a system (m) of a terminal station repeater REP1 is interrupted, an optical output interruption signal SOUT-ALM is transmitted. The other system at the transmission side superimposes the information that the system (m) transmits the SOUT-ALM onto the own transmission signal and transmits the result to a terminal station repeater REP2. An REC-ALM is transmitted from the optical reception panel ORm of the system (m) at the reception side and the optical reception panel of the other system receives the information that the optical transmission panel OSm transmits the SOUT-ALM. In this case, it is discriminated that the cause to the fault is the optical transmission panel OSm. In case of the transmission of the REC-ALM only from the optical reception panel, it is discriminated as the fault of an optical fiber.