A wavelength redundancy device for a wavelength multiplexing optical transmission system in which WDM (Wavelength Division Multiplex) line signals of different wavelengths are output from a plurality of working transponders (3d, 3e, 3f,) configured to process input client signals having different types of transmission rates, and there are a plurality of types of transmission rates of the WDM line signals, the wavelength redundancy device comprising: - optical couplers (2d, 2e, 2f) configured to output the client signals to the working transponders (3d, 3e, 3f,), and output redundancy client signals into which the client signals are branched; a transponder for redundancy, including a client interface, configured to process a signal input through the client interface and output the processed signal as a redundancy WDM line signal; and - a selector/switcher (42, 43, 44, 45, 80, 81, 82), characterized in that the selector/switcher (42, 43, 44, 45, 80, 81, 82) includes: an M: N switch (42,44,45) configured to selectively output a signal with a transmission rate non conforming to the client interface of the transponder for redundancy, among the redundancy client signals outputted from one of the optical couplers (2d, 2f), where M is an integer greater than or equal to 2 and N is an integer greater than or equal to 1; - a muxponder (80, 81, 82) configured to process the redundancy client signal outputted from the M: N switch (42, 43, 44, 45) and output the processed redundancy client signal as a redundancy client signal with the transmission rate conforming to the client interface of the transponder for redundancy; and - an N:1 switch configured to selectively output, to the transponder for redundancy, the signal outputted from the muxponder (80, 81, 82), and a signal with a transmission rate conforming to the client interface of the transponder for redundancy among the redundancy client signals outputted another of the optical couplers, wherein: when trouble has occurred in any of the wavelengths of the WDM line signals output from the working transponders (3d, 3e, 3f), the N:1 switch is adapted to select a redundancy client signal corresponding to the client signal input to the working transponder to output the WDM line signal of the wavelength where the trouble has occurred, and to output the selected redundancy client signal as a redundancy client signal with the transmission rate conforming to the client interface of the transponder for redundancy; and the transponder for redundancy is adapted to process the redundancy client signal outputted from the N:1 switch, and output the processed redundancy client signal as the redundancy WDM line signal. - optical couplers (2d, 2e, 2f) configured to output the client signals to the working transponders (3d, 3e, 3f,), and output redundancy client signals into which the client signals are branched; a transponder for redundancy, including a client interface, configured to process a signal input through the client interface and output the processed signal as a redundancy WDM line signal; and - a selector/switcher (42, 43, 44, 45, 80, 81, 82), characterized in that the selector/switcher (42, 43, 44, 45, 80, 81, 82) includes: an M: N switch (42,44,45) configured to selectively output a signal with a transmission rate non conforming to the client interface of the transponder for redundancy, among the redundancy client signals outputted from one of the optical couplers (2d, 2f), where M is an integer greater than or equal to 2 and N is an integer greater than or equal to 1; - a muxponder (80, 81, 82) configured to process the redundancy client signal outputted from the M: N switch (42, 43, 44, 45) and output the processed redundancy client signal as a redundancy client signal with the transmission rate conforming to the client interface of the transponder for redundancy; and - an N:1 switch configured to selectively output, to the transponder for redundancy, the signal outputted from the muxponder (80, 81, 82), and a signal with a transmission rate conforming to the client interface of the transponder for redundancy among the redundancy client signals outputted another of the optical couplers, wherein: when trouble has occurred in any of the wavelengths of the WDM line signals output from the working transponders (3d, 3e, 3f), the N:1 switch is adapted to select a redundancy client signal corresponding to the client signal input to the working transponder to output the WDM line signal of the wavelength where the trouble has occurred, and to output the selected redundancy client signal as a redundancy client signal with the transmission rate conforming to the client interface of the transponder for redundancy; and the transponder for redundancy is adapted to process the redundancy client signal outputted from the N:1 switch, and output the processed redundancy client signal as the redundancy WDM line signal. when trouble has occurred in any of the wavelengths of the WDM line signals output from the working transponders (3d, 3e, 3f), the N:1 switch is adapted to select a redundancy client signal corresponding to the client signal input to the working transponder to output the WDM line signal of the wavelength where the trouble has occurred, and to output the selected redundancy client signal as a redundancy client signal with the transmission rate conforming to the client interface of the transponder for redundancy; and the transponder for redundancy is adapted to process the redundancy client signal outputted from the N:1 switch, and output the processed redundancy client signal as the redundancy WDM line signal.