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.