PURPOSE: To perform the detection of an angular position with high accuracy by making the magnetic flux quantity of each pole of a rotor constant, by holding a ring-shaped permanent magnet magnetized to two poles in the axial direction thereof between two ring-shaped magnetic bodies having teeth in the axial direction from the pole surfaces of both sides of the permanent magnet over both inner and outer peripheral surfaces thereof.

CONSTITUTION: Magnetic fluxes pass the surface of the magnetic bodies 7 contacted with the pole surfaces of a permanent magnet 6 magnetized in the axial direction thereof from said pole surfaces to be uniformly dispersed to the teeth of the magnetic bodies 7. The teeth of the magnetic body 7 contacted with the N-side of the magnet 6 come to a N-pole and the teeth of the magnetic body 7 contacted with the S-side thereof come to a S-pole and both magnetic bodies 7 are arranged so that the teeth of two magnetic bodies become alternate and, therefore, the N-pole and the S-pole alternately appear on the circumferential surface of a rotor 2 and the circumferential surface becomes equivalent to the permanent magnet having many poles each having constant magnetic flux quantity. One set of two outside and inside Hall elements 3, 4 having the difference of an electric angle 90° allow a current I to flow and, if both elements are placed in magnetic flux B, output VH of B × I is generated.