To provide a vehicular theft detection device capable of reporting a user that the setting of an inclination sensor system must be changed 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 a vicinity of an indoor mobile type parking lot or a ferry landing based on the acquired position of the vehicle. When it is determined that the present position of the vehicle is in a vicinity of the indoor mobile type parking lot or the ferry landing, the CPU 11 outputs the reporting signal to report the user to turn off an inclination sensor system. Thus, when the user turns off an ON/OFF switch 38, the security ECU 1 executes the determination of presence/absence of occurrence of theft based on the output of other theft detection sensors than an inclination sensor 35 while a warning mode is continued.