PURPOSE: To easily detect a fault point of an optical fiber transmission line in light- wavelength division multiplex mode by providing a reflecting mirror for reflection to the side of the transmission line, and moving the movable mirror.

CONSTITUTION: The transmission line consists of one optical fiber 5, and an A-station side optical demultiplexer and multiplexer and a B-station side optical demultiplexer and multiplexer 6 between the side of a station A and the side of a station B. The optical demultiplexer and multiplexer has input ports 10 and 11 for wavelengths λ1 and λ2 and an output port 12 for a wavelength λ3, and is provided with a transmission line port 18, and the optical demultiplexer and multiplexer is further equipped with a reflecting mirror 13 which reflects light of wavelength λ1 totally for demultiplexing and multiplexing, a reflecting mirror 14 which reflects light of wavelength λ2 totally and transmits the light of wavelength λ1, and a reflecting mirror 15 which transmits light of wavelength λ3 and reflects the light of wavelengths λ1 and λ2 totally. Further, the reflecting mirror 16 which is provided on the side of the transmission line port 18 and reflects the light of wavelengths λ1, λ2, and λ3 totally moves to a position 17 shown by a broken line in case of a fault. In this case, the fault point of the transmission line 5 is known by observing the light of wavelength λ3 at the side of the station B or reflected light from the station B and the light of wavelengths λ1 and λ2 on the side of the station A.