To improve an S/N and transmission efficiency by providing a variable impedance circuit and a standing wave ratio detection part and controlling the impedance circuit in order to minimize the standing wave ratio based on the information of the standing wave ratio detection part.

A standing wave ratio detection part 5 detects the standing wave ratio of a power line, and a microcomputer 6 controls an impedance circuit 4 to minimize the detected standing wave ratio. If the impedance of the power line varies by some reason, a standing wave ratio caused by the impedance change is confirmed. Thus the part 5 detects this ratio and the computer 6 calculates impedance to minimize the standing wave ratio based on the detection information of the part 5. Then the computer 6 outputs the DC control voltage to secure a combination to which the impedance of the circuit 4 is most approximate. As a result, the S/N is never deteriorated by the impedance change of the power supply and high transmission efficiency is maintained.