PURPOSE: To extend power supply distance by controlling an impedance of a branch station, depending upon a stored voltage at the branch station that receives and rectifies AC power between an end of a metal piping and an earth electrode, which AC power a main station inputs between the other end of the metal piping and the earth electrode.

CONSTITUTION: A transmission equipment 4 of a main station system 3 closes contacts 43 and 44 of a relay 42 for a specified period of time by operation of a timer 41 and inputs AC power 2 between one end of a metal piping 1 (E.G. underground cable) and an earth electrode 5 by a transformer 45. A power receiving equipment 7 of a branch station system 8 connected between the other end of the metal piping 1 and an earth electrode 6 receives AC power through a variable impedance 71, which is rectified by the use of diodes D1 and D2, and a capacitor C1 to charge a capacitor C2. When a voltage of the capacitor C2 comes to have a specified value, a voltage detector 73 energizes a relay 74 to close contacts 75 and 76, and makes the branch station system 8 to be ready for receiving power. An impedance controller 72 varies the impedance 71 depending upon the voltage of the capacitor C2. This enables extending power supply distance and increasing the number of branch stations.