In a toll gate system in which the toll is automatically and electronically collected through wireless communication between an on-board electronic device and a stationary electronic device installed at the toll gate, illegitimate or fraudulent actions committed in the system are detected by the on-board device. When such actions are detected, a communication function of the on-board device is made inoperative. After disposing the illegitimate actions properly, the communication function of the on-board device is restored so that the on-board device can be used again thereafter. The illegitimate action such as opening the on-board device for changing or reading the data contained therein can be detected by sensing removal of screws fastening a circuit board to a case of the on-boar device.