Abstract: A transmitter 16, receiver 20 and control circuit 10are provided on a mobile unit, and a response circuit 26, 28, 30 and a control circuit 32, 34, 30 are provided on a portable unit. Priorities are assigned to the transmission of a recognition signal by the response circuit and the transmission of a recognition signal by the operation circuit, and by so doing the transmission from the operation circuit is given priority and performed first if both transmissions are requested simultaneously.

Abstract: A first transmitting antenna (10) transmits an ID request and a receiving antenna (18) receives a response from a portable device. The received response includes an ID, which is checked with IDs stored in a memory (28). If the checking is successful, the lock is opened by a door lock (30). Contrarily, if the checking fails several times continuously, transmission of the ID request from the first transmitting antenna (10) is suspended for a predetermined time. Thereby even if the ID request competes with the ID request from a second vehicle, the portable device can respond the ID request from the second vehicle. Transmission of an ID request is preferably inhibited by judging the request to be fraudulent when the checking fails several times continuously.

Abstract: A vehicle theft prevention device prohibiting release of a vehicle theft prevention control under a predetermined condition even when the control is applied using an authorized portable tool to thereby prevent the vehicle from running. Even when a vehicle ECU 14 controls to release theft prevention control applied by a theft prevention device including an engine control device 12a, a steering locking device 12b, and so on based on a specific code corresponding to a request code from an authorized portable tool 16, the specific code corresponding to the request code from the first portable tool 16 is invalidated upon receipt of a security request code. With this arrangement, an operation to release the theft prevention function of the theft prevention device is prohibited. Moreover, when release control is performed based another specific code (a security code) requiring a request code, vehicle theft can be prevented.