Three power supply devices 31 are connected in series via a transmission and receiving coupler 32 to the feeder 12, a short-circuit switch 38 that short-circuits both ends of each transmission coupler 32B is provided, a circuit including three receiving couplers 32A and the feeder 12 is set to a series resonant circuit at a predetermined frequency f, impedance of a circuit including each transmission coupler 32B, and a line connecting the power supply device 31 and the transmission coupler 32B is set to capacitive reactance at the predetermined frequency f, and an voltage to be output to the circuit including the line is controlled while a current flowing through the feeder 12 is being fed back to output a constant current of the predetermined frequency f to the feeder.