PURPOSE: To prevent misoperation by connecting a theft diagnostic device and a theft judging device by communication lines, and diagnosing non-theft in the case of no theft being judged by the judging device even after the lapse of the specified time from the actuation of the diagnostic device.

CONSTITUTION: In an automobile with theft preventing security function provided at a remote door lock system, a theft diagnostic device 11 formed of an immobilizer computer is connected to a theft judging device 19, formed of an engine control computer, by communication lines S1, S2. At the time of starting theft diagnosis by the diagnostic device 11, counting is performed by plural timers put in action by a program, and non-theft is diagnosed in the case of no judgment of theft being outputted from the judging device 19 even after the lapse of the specified time or more. In the case of the judgment of theft being outputted, the output of a driving signal is cut off to inhibit the start of an engine, and a warning lamp 24 is lighted to prevent theft. Misjudgment of theft caused by the misoperation or the like of the timer can thereby be prevented.