The magnetic flux density by magnetic poles of electromagnets (M1 to M12) arranged at the same place in the axial direction of a rotor (1) at predetermined intervals in the circumferential direction of the rotor (1) varies gradually in the circumferential direction of the rotor (1). Even though there is a difference between the maximum and minimum of the magnetic flux density, the variation of the magnetic flux density between the magnetic poles of adjacent electromagnets is small and smooth. The sensitivity is good and stable sensing result is obtained.
