A wavelength division-multiplexing system is provided that uses wavelength grids in conformity with the ITU-T Recommendation G.692 and that is realizable at lower costs. In the system, the wavelengths of signal light beams transmitted from plural optical transmitters are respectively spaced apart in such a manner that multiplexed wavelengths are set at regular intervals within the band range of the optical amplifier 5. The wavelength range of each laser beam covers at least two grids including neighboring grids. The wavelength demultiplexer 3 on the receiving side has the passband passing the wavelengths in a laser wavelength range corresponding to each channel.