To easily and surely detect the removal of a wheel from a vehicle.

Magnetism naturally accumulating in a space inside the vehicle on or near the wheel attached to the vehicle is detected by a magnetism sensor 11. A control part 21 calculates an average value of outputted sensor data based on the magnetism detected by the magnetism sensor 11 for each predetermined interval, and determines that the wheel is removed from the vehicle when the average value of the sensor data is stable in a value which is not a regular state after being varied to a predetermined threshold or higher, and controls an output part 13 and outputs a separation notification signal to the outside. The present invention can be applied to a vehicular wheel anti-theft device.