In a vehicular anti-theft security system which includes a theft detection sensor that detects an abnormality of the vehicle associated with vehicle theft, and performs a theft preventing operation when the theft detection sensor detects the abnormality, an altitude measuring device is provided for measuring the altitude of the vehicle, and a controller restricts the theft preventing operation in the case where the altitude measuring device observes a specified change in the altitude of the vehicle within a specified period of time when the theft detection sensor detects the abnormality.