PURPOSE: To power-energize a sub station without preparing a power source circuit provided with a battery or a power source transformer and a rectification circuit, etc., for the sub station so as to simplify constitution.

CONSTITUTION: A key station 1 is connected with plural stations 4 to 6 through a pair of transmission lines, operates by switching a power supply mode and a signal transmission mode and supplies direct power from one transmision line to the other in the power supply mode and power in the voltage mode so that the capacitors of the sub stations 4 to 6 are successively charged from the sub station near the key station 1. After finishing the charge of all the sub stations 4 to 6, in the signal transmission mode of the key station 1, direct current is ON/OFF-controlled in the direction opposite to the power supply mode to transmit/receive the signal in a current mode and a termination diode 7 which becomes a forward direction in the signal transmission mode is connected between terminals on the side opposite to the key station 1 of the sub station 6 which is the farthest from the key station 1 so as to operate signal transmission.