A system for counting changes of direction of a person, comprising a housing (BT) and fixing means (EF) for securely binding said housing (BT) to a body part of said person, characterized in that it comprises : - means for measuring the direction of the terrestrial magnetic field in the reference frame of the housing comprising a calibrated triaxial magnetometer capable of providing measurements of the terrestrial magnetic field at a given location; - means for determining (DET_INC) a quantity representative of the inclination of the magnetic field measured with respect to vertical, comprising a calibrated accelerometer with three measurement axes provided with fixing means (FIX) configured to keep said triaxial accelerometer in substantially constant relative orientation with respect to said magnetometer , and configured to determine said representative quantity of said inclination on the basis of the measurements of said accelerometer ; and - first detection means (DETEC1) for detecting a change of orientation of an orthogonal projection of said direction of the earth magnetic field in a horizontal planar reference frame of axes of constant directions connected in translation to said person, on the basis of said measured direction of the magnetic field and said quantity representative of the inclination of the magnetic field with respect to the vertical, comprising first producing means (ELAB1) for producing a signal representative of said change of orientation relative to a reference orientation corresponding to a reference instant (t ref ), said first elaboration means (ELAB1) comprising first estimation means (EST1) of the vector product (V) of the vector (A) of the measurement signals transmitted by the accelerometer and of the vector (B) of the measurement signals transmitted by the magnetometer , and configured to detect a change of sign of said vector product (V), and means for comparing the current value of the vector product (V(t)) with the value of the vector product at reference instant (V(t ref )). - means for measuring the direction of the terrestrial magnetic field in the reference frame of the housing comprising a calibrated triaxial magnetometer capable of providing measurements of the terrestrial magnetic field at a given location; - means for determining (DET_INC) a quantity representative of the inclination of the magnetic field measured with respect to vertical, comprising a calibrated accelerometer with three measurement axes provided with fixing means (FIX) configured to keep said triaxial accelerometer in substantially constant relative orientation with respect to said magnetometer , and configured to determine said representative quantity of said inclination on the basis of the measurements of said accelerometer ; and - first detection means (DETEC1) for detecting a change of orientation of an orthogonal projection of said direction of the earth magnetic field in a horizontal planar reference frame of axes of constant directions connected in translation to said person, on the basis of said measured direction of the magnetic field and said quantity representative of the inclination of the magnetic field with respect to the vertical, comprising first producing means (ELAB1) for producing a signal representative of said change of orientation relative to a reference orientation corresponding to a reference instant (t ref ), said first elaboration means (ELAB1) comprising first estimation means (EST1) of the vector product (V) of the vector (A) of the measurement signals transmitted by the accelerometer and of the vector (B) of the measurement signals transmitted by the magnetometer , and configured to detect a change of sign of said vector product (V), and means for comparing the current value of the vector product (V(t)) with the value of the vector product at reference instant (V(t ref )). <EOS>