An antitheft system can be provided by constructing such that, when a stop signal has been inputted from an engine 7, a main controller 2 outputs, until a first predetermined time Ts elapses, a turn-on signal to switches 8, 9 to perform a similar antitheft processing as in an operation of the engine 7 and after an elapse of the first predetermined time Ts, instructs the switch 9 to repeat a turn-on and turn-off at predetermined time intervals .DELTA.t, and, whenever the switch 9 is brought into a turned-on position, detects a position at that time by a position detecting controller 3 and compares the position with a position at the time of an engine stop to determine whether or not a theft has taken place. Owing to this construction, it is no longer necessary to always feed power to the position detecting controller 3 during the stop of the engine 7, thereby making it possible to cut down the feeding of as much power to the controller 3 as a time elapsed during which the switch 9 is kept off. The antitheft system can, therefore, minimize the power consumption of a control system during the engine stop.