To provide a vehicular theft detection device capable of automatically changing an inclination sensor system to an adequate detection mode by automatically determining the situation of a vehicle.

When a security ECU 1 is set in a warning mode, a CPU 11 acquires the position of a vehicle immediately before the ignition-off based on information from a navigation device 2, and determines whether or not the present position of the vehicle is in an indoor mobile type parking lot, a ferry landing place or in a vicinity thereof based on the acquired position of the vehicle. When it is determined that the present position of the vehicle is in an indoor mobile type parking lot, a ferry landing or in a vicinity thereof, the CPU 11 forcibly turns off an inclination sensor system, and displays on a monitor 24 of the navigation device 2 that the inclination sensor system is turned off. Malfunction of the inclination sensor system caused by the rolling of a pallet in the parking lot or a ferry can be prevented.