Abstract: The present invention relates to a disengagement detection device and method capable of detecting the disengagement of a wheel from a vehicle easily and reliably. A magnetic sensor 11 detects the magnetism naturally accumulated in a space inside a vehicle at or near a wheel mounted on the vehicle. A control unit 21 determines, for each prescribed interval, the average value of the output sensor data based on the magnetism detected with the magnetic sensor 11, judges that the wheel disengaged from the vehicle when the average value of the sensor data fluctuates by a prescribed threshold value or more and thereafter stabilizes at a value that is not in a stationary state, and controls the output unit 13 so as to output a disengagement notification signal to the outside. The present invention may be employed in antitheft devices for car wheels.