An antitheft apparatus, which is installed in a vehicle having an expandable body system to expand expandable body that confine a motion of a burglar in predetermined space portions in a compartment, ejects material as a source of foam polystyrene into the compartment and then injects a steam to the material to confine the motion of the intruder by the foam polystyrene when a presence of the intruder in the compartment is sensed.