First and third current paths are connected at mutual one end, and second and fourth current paths are connected at mutual one end. A transmitter sends signals utilizing the change of a current by changing the current flowing between other ends of the first and second current paths. A switching part brings about a non-conductive state between the other end of the third current path and the other end of the fourth current path by being applied with a control signal from a controller. By contrast with this, the switching part brings about a conductive state between the other end of the third current path and the other end of the fourth current path by not being applied the control signal.