It is an object to monitor signal flow (optically switched state) in an optical node without using a monitor light. In each of output ports of the optical node, a part of output signals is turned back, and the output signal light is subjected to intensity modulation or phase modulation, assigned port identification information, and allowed to reverse in the optical node. From an input port corresponding to the reversed output signal, a plurality of signals turned back are output. The plurality of signals are appropriately converted into intensity modulation from phase modulation and separated by a device having a spectroscopic function, and identification information is read out based on an intensity of a signal light for each signal, thereby determining an optically switched state to an output port corresponding to the input port.