A transmitting device, includes inputting a multiplex light multiplexed a first wavelength-multiplexed signal light stream in a first wavelength band and a second wavelength-multiplexed signal light stream in a second wavelength band; inputting a multiplex light multiplexed a third wavelength-multiplexed signal light stream in a first wavelength band and a fourth wavelength-multiplexed signal light stream in a second wavelength band; converting the first wavelength-multiplexed signal light stream to the second wavelength band; converting the third wavelength-multiplexed signal light stream to the second wavelength band; generating a first output signal light multiplexed by signal light in a first wavelength band among the multi-wavelength light so that wavelengths do not overlap; generating a second output signal light multiplexed by signal light in a second wavelength band among the multi-wavelength light so that wavelengths do not overlap; converting the first output signal light to the first wavelength band; and outputting the multiplexed light.