A wavelength-division multiplexing transmission device including a receiver and a transmitter, the receiver being configured to demultiplex first wavelength-division multiplexed light received by the device from a first transmission line (a) and configured to demodulate one of the demultiplexed light to output reception data while passing another one of the demultiplexed light through the receiver, the transmitter being configured to transmit, to a second transmission line (b), second wavelength-division multiplexed light generated by multiplexing light of wavelengths which have been modulated based on transmission data and light of a wavelength passed through by the receiver, the wavelength-division multiplexing transmission device comprising: - a dummy light source configured to emit and quench dummy light;- a monitoring unit (26; 154) configured to monitor an optical level relating to the received first wavelength-division multiplexed light; and - a dummy light controller configured to control the dummy light source to emit dummy light in case where the monitoring unit (26; 154) determines based on the monitored optical level that the first wavelength-division multiplexed light is in a condition of input interruption; characterized by further comprising: - a first multiplexer configured to multiplex the wavedivision multiplexed light that has been passed through by the receiver and the dummy light from the dummy light source, and configured to output light obtained by the multiplex; - a wavelength filter configured to block light output by the first multiplexer, which has a wavelength to be terminated at the wavelength-division multiplexing transmission device, and configured to allow light output by the first multiplexer to pass through, which has a wavelength not to be terminated at said device; and - a second multiplexer configured to multiplex the light of the wavelength modulated based on the transmission data and the dummy light passing through the wavelength filter after being emitted by the dummy light source, and configured to output wavelength-division multiplexed light obtained by the multiplex, wherein the transmitter transmits, to the second transmission line (b), wavelength-division multiplexed light as the second wavelength-division multiplexed light which has been amplified after being output by the second multiplexer. - a dummy light source configured to emit and quench dummy light;- a monitoring unit (26; 154) configured to monitor an optical level relating to the received first wavelength-division multiplexed light; and - a dummy light controller configured to control the dummy light source to emit dummy light in case where the monitoring unit (26; 154) determines based on the monitored optical level that the first wavelength-division multiplexed light is in a condition of input interruption; - a first multiplexer configured to multiplex the wavedivision multiplexed light that has been passed through by the receiver and the dummy light from the dummy light source, and configured to output light obtained by the multiplex; - a wavelength filter configured to block light output by the first multiplexer, which has a wavelength to be terminated at the wavelength-division multiplexing transmission device, and configured to allow light output by the first multiplexer to pass through, which has a wavelength not to be terminated at said device; and - a second multiplexer configured to multiplex the light of the wavelength modulated based on the transmission data and the dummy light passing through the wavelength filter after being emitted by the dummy light source, and configured to output wavelength-division multiplexed light obtained by the multiplex,