An anti-theft system for a vehicle includes an on-vehicle machine including reporting means configured to report by radio an illegal activity affecting the vehicle to a designated opponent to be reported to in a case where the illegal activity is detected; and an external facility, the external facility including obstruction radio wave detection means configured to detect a radio wave obstructing the radio-report by the reporting means; and illegal activity prevention means configured to prevent the illegal activity in a case where the radio wave is detected by the obstruction radio wave detection means.