An antitheft device is provided, whereby a person who attempts to commit a theft of a vehicle can be captured, having an improved crime prevention performance to thefts of vehicles, electrical equipment mounted on vehicles, money and articles left inside vehicles and the like. The antitheft device includes an expansible body system to expand an expansible body for blocking the motion of an intruder in a prescribed space inside a vehicle, which emits a substance to be a base of foaming polystyrene inside the vehicle and applies steam to the substance so that the foaming polystyrene blocks the motion of an intruder, when the presence of the intruder inside the vehicle is detected.