A communication method, for use in a communication system that performs radio communication between a number of vehicle-mounted terminal devices and a base station device, includes transmitting, by each of the vehicle-mounted terminal devices, vehicle-mounted device information thereof, and lastly-accessed identification information indicating a base station device that was in the radio communication with a vehicle-mounted terminal device just before when the communications therewith is cut off and authenticating, by the base station device, when there is a match between the transmitted lastly-accessed identification information and the identification information of the base station device stored in the base station device, based on the vehicle-mounted device information of the vehicle-mounted terminal devices stored in the base station device not entirely but partially, and the vehicle-mounted device information transmitted by each of the vehicle-mounted terminal devices.