PURPOSE: To transmit stably a high-frequency carrier signal on an AC power line by connecting a signal transmission unit transmitting a high frequency carrier signal in superposition to an AC voltage waveform to an AC power line.

CONSTITUTION: A primary winding 7 of a transformer 6 is connected to the AC power supply 5 connected to a commercial AC power supply 4, and an AC power line 10 is connected to a secondary winding 9 of the transformer 6 via a block filter 8. Then, signal transmission units 11, 12 transmitting a high- frequency carrier signal in superposition to the AC voltage waveform are connected to the AC power line 10. The unit 11 consists of a transmitter T1 and a receiver R2, and the unit 12 consists of a receiver R1 corresponding to the transmitter T1 and a transmitter T2 corresponding to the receiver R2. Thus, since the high-frequency carrier signal is not attenuated by the secondary winding of the transformer 6 and the degree of the attenuation is not fluctuated, the high- frequency carrier signal is transmitted stably on the AC power line 10.