PROBLEM TO BE SOLVED: To monitor a light signal which is switched in the state of light as it is and which passes through at a low cost. SOLUTION: A device 207 for monitoring the signal is connected to a part of the output terminal of an optical switch circuit network 201. Only when an instruction to monitor the light signal is received, the transmitted light signal is connected to the monitor device 207 and signal quality is monitored. A device for terminating the overhead of the light signal can be used as the monitor device 207, and a bit error rate can be checked and an identifier mentioned in the overhead can be recognized. Thus, all the light signals can be monitored only by preparing few monitor devices for the multiple light signals. Thus, a main signal can inexpensively be monitored. Since the signal is not always monitored but it is monitored only when a monitoring request is given, management information quantity of the device controlling a node is reduced and an inexpensive system can be constructed.