a plurality of optical senders including one or more fixed-wavelength optical senders each outputting light having a fixed-wavelength different from other of said plurality of optical senders and one or more variable-wavelength optical senders;

optical coupling means having a plurality of optical input ports, to each of which light is input from one of said optical senders, and a plurality of optical output ports, from each of which light is output to one of said optical receivers, said optical coupling means for coupling light from said plurality of optical senders and outputting a coupled light from each of the plurality of optical output ports; a plurality of optical receivers including one or more fixed-wavelength optical receivers inputting light from one of the optical output ports and receiving light of a fixed-wavelength different from each other and one or more variable-wavelength optical receivers; and

control means which controls transmission timing in said optical senders in such a manner that light at an identical wavelength is not simultaneously input from different ones of said optical senders to said optical coupling means, wherein:

a combination of both one of said variable-wavelength optical receivers and one of said variable-wavelength optical senders converting such input light, input to one of the optical input ports, into light of a reception wavelength receivable by one of said fixed-wavelength optical receivers and reinputting the light at said reception wavelength to said optical coupling means,

each variable-wavelength optical receiver inputs light from one of the optical output ports and receives light of a wavelength variably-tuned among wavelengths of light and outputs a signal based on the received light,

each variable-wavelength optical sender inputs the signal from one of the variable wavelength optical receivers and outputs light of a wavelength variably-tuned among wavelengths received by said fixed-wavelength optical receivers to the optical input port, and

said control means staggers transmission timing for two or more of said optical senders, including one of said variable-wavelength optical senders, so that light at an identical wavelength is not simultaneously input from said fixed-wavelength optical sender and said variable-wavelength optical sender to said fixed wavelength optical receiver.