To provide a vehicular theft preventing device and a theft preventing method for reducing a malfunction when a man-caused action which is not an illegal act is detected.

The vehicular theft preventing device 10 transits to a warning state for performing at least either warning or reporting when a theft action to a vehicle is detected. The theft preventing device 10 is characterized by comprising a vehicular situation detecting means 23 for detecting a vehicular situation, an illegal act determining means 31 for determining whether the vehicular situation detected by the vehicular situation detecting means 23 is man-caused or not, a circumferential state detecting means 24 for detecting a state around the vehicle, and an illegal act deciding means 32 for deciding the illegal act based on the state around the vehicle detected by the circumferential state detecting means 24 when the illegal act determining means 31 determines it is the man-caused vehicular situation.