The on-vehicle charger and vehicle charging system

A vehicle-mounted charging device (12) comprises a vehicle-mounted battery (18), a vehicle-mounted electronic control unit (30) and a modem start control unit. The vehicle-mounted battery is charged via a charging cable (16) with power, which is supplied from an external charging device. The vehicle-mounted electronic control unit is designed so that it performs a charging communication with the external charging device relative to the charging of the vehicle-mounted battery by using an in-band modem (32). The modem start control unit starts the in-band modem when the vehicle-mounted electronic control unit performs the charging communication, and last for starting the in-band modem when the vehicle-mounted electronic control unit does not perform the charging communication.