To provide a theft preventing device including an electric switch to detect insertion of a key.

In a theft preventing device 10 for an automobile including an electric switch 34 to an insertion direction (from the back side forward, usually) for a key into the theft preventing device 10 through action of a pusher 50 to excite the electric switch 34 under action of the key which acts to an elastic means 38 slidable in both directions to a key insertion axial line and having a tendency to move the pusher 50 backward to a stopper at a retraction stop position in pulling the key, the pusher 50 freely slides in a housing 52 of a main body including a means to slide and guide a forward end part of the pusher 50, and a back end part of the pusher 50 on which the key acts when the key is inserted cooperates with a means on the main body forming an inclination part to move the back end part in an inclination direction relatively to a slide direction between a lower retraction position corresponding to the stop position of the pusher 50 and a higher forward excitation position.