A branching unit includes a supply destination switching section that switches one supply destination of each of a plurality of power lines that supply electric power to a unit that transfers an optical signal received from a terminal station unit through a transmission line to another supply destination corresponding to a control signal received from the terminal station unit; a current detection section that outputs a detection signal that includes information that denotes whether currents are flowing in the plurality of power lines and information that represents current directions if currents are flowing; and a monitor section that transmits a monitored result based on at least one of a plurality of detection signals received from the current detection section to the terminal station unit.