A first optical transmission apparatus comprising: a wavelength multiplexing device configured to multiplex signal lights and output the light to a wavelength multiplexing unit; a light source amplifier configured to output spontaneous emission light; a first wavelength demultiplexing device configured to demultiplex the spontaneous emission light into sub-band dummy lights and to output the sub-band dummy lights to the wavelength multiplexing unit, wherein each sub-band includes a plurality of wavelengths of the signal lights; a second wavelength demultiplexing device configured to demultiplex the spontaneous emission light into channel dummy lights having the same wavelengths as the signal lights, and to output the channel dummy lights to the wavelength multiplexing unit; the wavelength multiplexing unit configured to wavelength multiplex the signal lights output by the wavelength multiplexing device together with dummy light selected among the outputs of the first and the second wavelength demultiplexing devices; a third wavelength demultiplexing device configured to receive a wavelength multiplexed light from a second optical transmission apparatus, and to divide the wavelength multiplexed light into lights having a plurality of wavelengths; light receiving units (46, 47) configured to receive the lights having the plurality of wavelengths from the third wavelength demultiplexing device, to extract signal-to-noise ratio information from the signal lights, and output the signal-to-noise ratio information to a selecting unit; and the selecting unit configured to determine the state of a sub-band based on the signal-to-noise ratio information and according to determination results, to select, for each of the sub-bands, any one among the channel-specific dummy light and the sub-band-specific dummy light, wherein the wavelength multiplexing unit is configured to wavelength multiplex the signal lights together with the dummy light selected for each sub-band by the selecting unit. a wavelength multiplexing device configured to multiplex signal lights and output the light to a wavelength multiplexing unit; a light source amplifier configured to output spontaneous emission light; a first wavelength demultiplexing device configured to demultiplex the spontaneous emission light into sub-band dummy lights and to output the sub-band dummy lights to the wavelength multiplexing unit, wherein each sub-band includes a plurality of wavelengths of the signal lights; a second wavelength demultiplexing device configured to demultiplex the spontaneous emission light into channel dummy lights having the same wavelengths as the signal lights, and to output the channel dummy lights to the wavelength multiplexing unit; the wavelength multiplexing unit configured to wavelength multiplex the signal lights output by the wavelength multiplexing device together with dummy light selected among the outputs of the first and the second wavelength demultiplexing devices; a third wavelength demultiplexing device configured to receive a wavelength multiplexed light from a second optical transmission apparatus, and to divide the wavelength multiplexed light into lights having a plurality of wavelengths; light receiving units (46, 47) configured to receive the lights having the plurality of wavelengths from the third wavelength demultiplexing device, to extract signal-to-noise ratio information from the signal lights, and output the signal-to-noise ratio information to a selecting unit; and the selecting unit configured to determine the state of a sub-band based on the signal-to-noise ratio information and according to determination results, to select, for each of the sub-bands, any one among the channel-specific dummy light and the sub-band-specific dummy light, wherein the wavelength multiplexing unit is configured to wavelength multiplex the signal lights together with the dummy light selected for each sub-band by the selecting unit.