The invention relates to a device for blocking a motor vehicle steering shaft (1) and preventing the rotation thereof by means of a blocking bolt (2) interacting with the blocking slots (3) of the steering shaft (1). A blocking pin carries out a reciprocating motion between the blocking position and a releasing position thereof by means of a control element (15) which is rotated in two directions by an electric motor (14) and interacts with a rotating position sensor (18). According to said invention, the control element (15) comprises a helical rib (39) or a helical groove which winds around the axis of rotation (22) of the control element (15) and interacts with the rotating position sensor (18) in order to determine the rotating position of said control element (15) corresponding to the releasing position of the blocking bolt (2) and the rotating position of the control element (15) corresponding to the blocking position of the blocking bolt (2) and another sensor (40) interacting with the blocking bolt (2) for determining the releasing position thereof.