An optical switch having a plurality of ports outputs an optical signal received from a first port that is one of the plurality of ports connected to an optical communication device from a second port that another one of the plurality of ports according to a transmission path of an optical signal. An optical branching part branches an optical signal which has been output from a second port in accordance with a branching ratio. A measurement part measures a round trip time by transmitting and receiving an optical signal to and from an optical communication device through an optical switch, and calculates a transmission distance of the optical signal on the basis of the measured round trip time. An instruction part instructs, the optical branching part, of the branching ratio determined on the basis of the calculated transmission distance.