An add-on communication device attached to a cable-mounted charging control device integrated with a cable assembly for charging an electric vehicle includes a control unit and a plurality of communication modules for communicating with a terminal device. The control unit determines a first communication module for transmitting a message, from among the plurality of communication modules. The control unit determines a second communication module for retransmitting the message, from among the plurality of communication modules excepting the first communication module, when reception of an acknowledgement message indicating normal reception of the message by the terminal device is abnormal.