To judge a regular key from a key in a short time even in the case where it is judged that the key is not the key in at least first collation processing by communication interference or the like in spite that the regular key is used.

An immobilizer ECU 30 transmits questionnaire data to the transponder T1 of a key K1, answer data are received from the transponder T1, self answer data are prepared from the questionnaire data and key K1 discrimination data, and one time collation processing is performed until both the answer data are collated to judge whether it is a regular key or it is repeated until the prescribed frequencies are completed. Since the transmission of the questionnaire is started in next time key collation processing immediately after the immobilizer ECU 30 receives the answer data from the transponder T1 in this time key processing, a cycle in which it is judged whether it is the regular key is shortened as compared with a case where the transmission of the next time questionnaire data is stated after the judgment in which the key is not the regular key.