An antitheft system for use in a vehicle includes an angle sensor for outputting a signal that represents a body angle of the vehicle, an angle variation determination unit for determining an angle variation of the body angle at a predetermined interval based on the signal outputted from the angle sensor, a theft detection unit for determining whether the vehicle is in a theft condition based on a comparison of the angle variation with a predetermined value, and a temperature sensor for sensing temperature of the angle sensor. The angle variation determination unit compensates the signal from the angle sensor based on the temperature of the angle sensor sensed by the temperature sensor before determining the angle variation.