The present invention teaches an antitheft device whereby the detection accuracy of an unauthorized intruder into a vehicle can be improved, comprising a section for judging whether or not there is an approaching object T toward a vehicle M based on information obtained from a surround monitoring sensor for monitoring the surround of the vehicle M, a section for judging whether or not the approaching object T toward the vehicle M is a suspicious person based on information obtained from a motion sensor for monitoring the periphery and/or the inside of the vehicle M when it is judged that there is an approaching object T toward the vehicle M, and a section for conducting processing such as lighting-on of an indicator when the approaching object T toward the vehicle M is judged to be a suspicious person.