To drastically reduce the crostalk of a high density wavelength division multiplexing system and to obtain the improved scalability for supporting many channels.

This optical demultiplexer divides the total number of input optical channels into the optical channels of a small group and supplies them to the separate pieces of demultiplexer modules 310 and 311. In the separate pieces of the demultiplexer modules 310 and 311, the optical channels of the group smaller than them are demultiplexed and the respective optical channels are filtered at the part of the output of the respective demultiplexer modules 310 and 311. The optical demultiplexer is made into the module and a smaller device footprint is used.