An ETC (electronic toll collection) system includes an antenna having a predetermined directivity for providing a limited radio-communication service zone. A vehicle sensor operates for detecting a vehicle which reaches a predetermined position in the limited radio-communication service zone. A radio signal is transmitted via the antenna. A decision is made as to whether or not a radio response to the radio signal is received via the antenna. In cases where a radio response to the radio signal is received, it is judged that there is an ETC vehicle incoming. In cases where the vehicle sensor detects a vehicle while a radio response to the radio signal is not received, it is judged that there is a non-ETC vehicle incoming.