PURPOSE: To effectively prevent the theft in a car by installing an antenna part forming a high-frequency electromagnetic field in the vicinity of a car door and allowing an alarm means to operate by detecting the variation of the impedance of the antenna part.

CONSTITUTION: When a driver gets-off from a car, he closes a door A and turns-ON a switch 7 installed into a trunk. Therefore, a high-frequency oscillation circuit 1 starts operation, and the high-frequency electric power is supplied into an antenna part 4. Therefore, a high-frequency electromagnetic field is formed in the vicinity of the car door A. When a man approaches the car door A in this state, the impedance of the antenna part 4 varies. The variation of the impedance is detected by a detecting circuit 5, which transmits an alarm signal 5a to operate an alarm means 6.