To improve balance of a power line communication line, and to suppress a radiation noise generated from the power line communication line.

A coupler circuit 40 is connected through a balance improving circuit 2 to lines 1L and 1N of a power line communication line 1. The balance improving circuit 2 is provided with a transformer 10 and high-pass filters 21 and 22. The transformer 10 is provided with coils 11 and 12 connected to each other with the same number of turns. One end of the coil 11 is connected through the high-pass filter 21 to the line 1L, and one end of the coil 12 is connected through the high-pass filter 22 to the line 1N. The coupler circuit 40 is provided with a coil 41 to which an unbalance signal is inputted and a coil 42 connected to the coil 41. The respective ends of the coil 42 are connected through the high-pass filters 21 and 22 to the lines 1L and 1N. The middle point of the coil 42 and the respective ends of the coils 11 and 12 are mutually connected and grounded.