A terminating device enables power line communication to be performed while suppressing degradation of the communication quality without decreasing the communication efficiency. The terminating device includes power supply plugs, a terminating circuit, a changeover switch, and power jack terminals, which are housed in an adapter case. The terminating circuit has an impedance corresponding to the impedance between power lines in the used frequency band of power line communication and is connected or disconnected between the power supply plugs by the changeover switch.