In a wavelength-division multiplexing transmission technique, a loss difference compensator for compensating a loss difference of an optical signal at each wavelength generated in a wavelength-division-multiplexed optical signal due to a wavelength-dependent transmission loss characteristic of an optical transmission line is disposed to the optical transmission line for transmitting the wavelength-division-multiplexed optical signal having a wide wavelength band in which optical signals at a plurality of wavelengths are wavelength-division-multiplexed to compensate the loss difference of the optical signal at each wavelength generated in the wavelength-division-multiplexed optical signal due to the wavelength-dependent transmission loss characteristic of the optical transmission line and a dispersion compensator, thereby transmitting the wavelength-division-multiplexed optical signal in a wide wavelength band for a long distance.