Power supply line communication modem and power supply line communication system

A power-line communications system comprising:

• a power line;
• a communications apparatus performing communications through the use of the power line;
• and
• a modem for power-line communications provided between the communications apparatus and the power line, the modem incorporating:
• a first input/output connected to the power line and performing input and output of power-line communications signals from and to the power line;
• a second input/output connected to the communications apparatus and performing input and output of data signals from and to the communications apparatus;
• a modem main body that generates a power-line communications signal by modulating carrier waves based on a data signal received at the second input/output and outputs the power line communications signal to the first input/output, and that demodulates a data signal from a power-line communications signal received at the first input/output and outputs the data signal demodulated to the second input/output;
• and
• common mode filters one of which is provided between the modem main body and the first input/output and the other of which is provided between the modem main body and the second input/output.