An embodiment of the invention is an optical communication system including: a plurality of interface boards which transmit and receive optical signals to and from interface boards facing the plurality of interface boards; and a monitoring control device which monitors states of the plurality of interface boards. A first interface board of the plurality of interface boards includes: a replacement unit capable of monitoring the states of the plurality of interface boards on behalf of the monitoring control device and independently receiving supply of power; and a control unit configured to start the replacement unit in a case where a fault occurs in the monitoring control device and stop or halt the replacement unit in a case where there is no fault in the monitoring control device.