An optical transmission apparatus outputs a main signal. An optical transmission apparatus superimposes a monitoring signal on an optical signal and outputs it. A submarine branching apparatus includes a return unit configured to return the monitoring signal received from the optical transmission apparatus and is configured to switch an output destination of the main signal received from the optical transmission apparatus to an optical transmission apparatus or the optical transmission apparatus. The optical transmission apparatus is configured to detect the monitoring signal returned from the return unit and notifies the optical transmission apparatus of a result of the detection. The optical transmission apparatus instructs the submarine branching apparatus to switch the output destination of the main signal in accordance with the notification.