Abstract: The invention relates to a device for detecting the position of a magnet which is positioned so that it is moveable (3, 23, 24) to produce a magnetic field (6) through a ferromagnetic wall (1, 29), a magnetic field sensor (10, 21, 21′) being situated in front of said wall. When said magnet (3, 23, 24) is moved in relation to said wall (1), the lines (6) of the field build up a useful flux (6) which progresses in said wall (1, 29). Said useful flux has magnetic remanence (7) which is retained after the magnet (3, 23, 24) has passed by, is aligned along the movement axis of the magnet (3, 23, 24) in accordance with the polarity, and builds up a fringing flux (12) to the front side of the wall (1, 29) where the magnetic field sensor (10, 21, 21′) is positioned. Outside of said wall, the fringing flux (12) is opposed and is aligned with the useful flux (6).