The present invention aims to provide a WDM optical communication system that can arrange efficiently optical signals of a plurality of bit rates at different wavelength spacing. To this end, for example, a basic constitution of a demultiplexing section to be applied to the WDM optical communication system of the present invention is provided with a demultiplexer that demultiplexes WDM signal light into 2m components in accordance with a plurality of wavelength regions set according to minimum wavelength spacing, 2m optical switches, each of which has one input port to which light from the demultiplexer is input, and a plurality of output ports, that selects one of the plurality of output ports to connect it to the input port, and m multiplexers that each multiplexes each output light of the 2m optical switches, wherein switching operations of the optical switches is set such that the bandwidth of each optical signal is approximately the same as the wavelength arrangement spacing corresponding to each bit rate.