To resolve the problems of the vehicle-to-roadside communication and the vehicle-to-vehicle communication, and to enhance the safety of the road traffic and the convenience of the user by expanding the communication providing area and further stabilizing the communication. A vehicle-to-roadside and vehicle-to-vehicle communication system including a plurality of roadside communication devices being arranged on a roadside with a narrow range communication area, and an on-board communication device mounted on a vehicle which travels with passing through the communication area, and carrying out a wireless communication by cooperating a vehicle-to-roadside communication and a vehicle-to-vehicle communication is provided; where the roadside communication device and the on-board communication device are configured to be mutually communicable through two wireless channels, and the on-board communication device is configured to execute channel switching after notifying channel switching to a communicating party via wireless channel in communication when switching the channel from one to the other.