Abstract: Roadside wireless devices are installed at roadside positions corresponding to the traffic lanes entering an intersection. Roadside wireless devices are also installed at roadside positions corresponding to the traffic lanes exiting an intersection. The roadside wireless devices on the entrance side receive an identification number from vehicle-mounted wireless ID devices of vehicles entering the intersection and transmit them to a roadside traffic monitoring device. Thereafter, the roadside wireless devices on the exit side receive an identification number from the vehicle-mounted wireless ID devices of vehicles exiting the intersection and transmit them to the roadside traffic monitoring device. From the identification numbers received, the roadside traffic monitoring device is able to determine the entrance from which a vehicle judged to be the same one had entered and the exit from which it exited, along with the type of vehicle.