To suppress a transmission loss of a power line communication signal, and make it easy to acquire approval of safety specifications of a modem used as a load.

The modem is provided with a voltage conversion means 11 which converts voltage of a commercial power line to AC voltage or DC voltage which is lower voltage than the source, and supplies to the modem 20; a transformer for signal transmission 12; two input terminals 10a, 10b which input the commercial power line voltage; and two output terminals 10c, 10d which output the AC voltage or the DC voltage of lower voltage; connects the input side of the voltage conversion means 11 to the input terminals 10a, 10b through a 1st high impedance means 13, 13; connects the output side to the output terminals 10c, 10d through a 2nd high impedance means 14, 14; connects a primary winding of the signal transmission transformer 12 to the input terminals 10a, 10b; and connects the secondary winding to the output terminals 10c, 10d.