An optical communications system which offers flexible and efficient cross-connect capabilities without using optical switch modules. With external commands, a variable wavelength setting unit varies output wavelength of each transmission unit. It also provides an output wavelength description indicating which wavelengths can be produced. An optical signal transmitter transmits an outgoing optical signal with the controlled wavelength, while an optical signal receiver receives an incoming optical signal with a single particular wavelength which have been produced through WDM processes. In a WDM unit, a WDM controller controls optical multiplexers and demultiplexers, and the resultant signals go out through optical output ports which are each associated with particular output wavelengths. In a monitoring station, an administration controller manages administrative data concerning connections and operations of transmission units, and based on that data, an optical cross-connect controller set up connections via the WDM unit, sending commands to the variable wavelength setting unit.