A data coupler includes an antenna substrate that is not electrically connected in a DC arrangement to an electric power line, is arranged adjacent to the electric power line, and is connected to a modem. A resonant circuit including an inductance element and a capacitance element is provided in the antenna substrate. In the resonant circuit, the inductance element is magnetically coupled to the electric power line. The resonant circuit transmits a radio-frequency signal superimposed on the electric power line to the modem. The resonant circuit also transmits a transmission signal from the modem to the electric power line.