System for measuring the axial movement of a mobile part rotating on an axis A, comprising: - a magnetic coding device, comprising at least one annular track fixed to the mobile part and on which are magnetized a plurality of north and south pole pairs (3, 4), equidistant, exhibiting a helical form of axis A; - a magnetic field detection device comprising at least one first and one second fixed magnetic sensor, arranged at a non-zero axial distance d from one another, radially opposite and at a reading distance from the peripheral track of the coding device and sensitive to the variations of the magnetic field induced by the coding device; and - a signal processing unit; characterized in that: - the first sensor comprises: - a first plurality of sensitive elements situated in a same plane at right angles to the axis A, the sensitive elements being divided into at least two first subsets; and - an electronic circuit arranged to deliver two analogue signals SIN1 and COS1 of the same amplitude in quadrature as a function of the signals from the first subsets; and - the signal processing unit comprises: - a computer for determining a signal ARCTAN1 = arctan (SIN1/COS1) from the signals SIN1 and COS1, and - an operator for determining a phase shift between the magnetic field seen by the first sensor and the magnetic field seen the second sensor, as a function of the signal ARCTAN1. - a magnetic coding device, comprising at least one annular track fixed to the mobile part and on which are magnetized a plurality of north and south pole pairs (3, 4), equidistant, exhibiting a helical form of axis A; - a magnetic field detection device comprising at least one first and one second fixed magnetic sensor, arranged at a non-zero axial distance d from one another, radially opposite and at a reading distance from the peripheral track of the coding device and sensitive to the variations of the magnetic field induced by the coding device; and - a signal processing unit; - the first sensor comprises: - a first plurality of sensitive elements situated in a same plane at right angles to the axis A, the sensitive elements being divided into at least two first subsets; and - an electronic circuit arranged to deliver two analogue signals SIN1 and COS1 of the same amplitude in quadrature as a function of the signals from the first subsets; and - a first plurality of sensitive elements situated in a same plane at right angles to the axis A, the sensitive elements being divided into at least two first subsets; and - an electronic circuit arranged to deliver two analogue signals SIN1 and COS1 of the same amplitude in quadrature as a function of the signals from the first subsets; and - the signal processing unit comprises: - a computer for determining a signal ARCTAN1 = arctan (SIN1/COS1) from the signals SIN1 and COS1, and - an operator for determining a phase shift between the magnetic field seen by the first sensor and the magnetic field seen the second sensor, as a function of the signal ARCTAN1. - a computer for determining a signal ARCTAN1 = arctan (SIN1/COS1) from the signals SIN1 and COS1, and - an operator for determining a phase shift between the magnetic field seen by the first sensor and the magnetic field seen the second sensor, as a function of the signal ARCTAN1.