Abstract: An output circuit sends out, to an input circuit, a control pilot signal generated by a voltage source. A voltage transformer is provided to a control pilot line on the output side of the output circuit, and a communication unit transmits and receives communication signals via the voltage transformer. A voltage transformer is provided to the control pilot line on the input side of the input circuit, and a communication unit transmits and receives the communication signals via the voltage transformer. A low-pass filter is provided to the control pilot line between the input circuit and the voltage transformer.