To provide a vehicle detection system which is usable to prevent the theft of an automobile or search the parking position of the automobile at low cost and further can rapidly respond to the theft of the automobile even when the parking position is short distance.

This vehicle detection system comprises an on-vehicle unit with a radio communication means, which is mounted on the automobile, and a portable unit with a radio communication means and a reporting means. In this system, when the communication between the on-vehicle unit and the portable unit by the radio communication means is cut, the reporting means is operated to report it (step S7), and a gain adjustment processing (step S3) for adjusting the transmitting output or receiving sensitivity of the radio communication means of the on-vehicle unit or the radio communication means of the portable unit in the state where the on-vehicle unit is distant from the portable unit is performed.