PURPOSE: To improve the reliability of signal transmission by separating power lines in terms of high frequencies not belonging to the control region from a main power line.

CONSTITUTION: A power line carrier unit 5 superimposing a high frequency carrier signal on a power line 4 so as to transmit the signal is connected to ≥2 power lines 4 connected to the load side of different main breakers 2, and a block filter 6 blocking the passing of the high frequency carrier signal is inserted between a branch breaker 3 and the load side of the main breaker 2 to the load side of which no power line carrier unit 5 is connected. Then the power line carrier unit 5 is connected to the load side of the main breakers 2 on the 1st and 4th floors and the block filter 6 is connected to the load side of the main breakers 2 on the 2nd and 3rd floors. Thus, the control coverage by the power line carrier system is limited to the 1st and 4th floors, and load equipments at the outside of the control coverage installed on the 2nd or 3rd floor do not give any adverse effect onto the signal transmission of the high frequency carrier signal in the main power line 1.