PURPOSE: To apply signal transmission between a master and a slave station by a low voltage distribution line carrier only by installing an interlocked earthing line up to the master station via an earth coupler with a high impedance to a commercial frequency and with a low impedance for a signal frequency.

CONSTITUTION: The master station 2 and the slave stations 9a, 9b transmit/ receive a signal through the interlocked earthing line 7 grounded via the earth couplers 8a, 8b, 8c showing a low impedance to the commercial frequency and showing a high impedance to the signal frequency. That is, the interlocked earthing line 7 is installed from a station 1 until earth lines 6a, 6b of the secondary winding of each pole transformer. Thus, the station 1 transmits/receives a signal with the slave stations by means of the low voltage distribution line carrier only through the interlocked earthing line 7, while not using any high voltage distribution line or any relay station.