An on-board communication device and a cooperative road-to-vehicle/car-to-car communication system that are adaptable to a road-to-vehicle communication system and a car-to-car communication system. The on-board communication device includes a car-to-car communication transfer service processing section, a car-to-car communication management service processing section, an application processing section, a transaction managing section, a transfer service processing section, a transmission/reception service processing section, and a transmission/reception service managing section, in which the car-to-car communication transfer service processing section and the car-to-car communication management service processing section include interfaces to the transfer service processing section that is an existing road-to-vehicle communication protocol.