To improve the detection accuracy of a rotational reference position.

A permanent magnet 22 as a rotor 15 is constituted of a cylindrical magnet that is magnetized to a plurality of N poles and S poles alternately in the circumferential direction, and a detecting part 26 is provided solidly at a part of the side end surface of a Hall element. The detecting part 26 is made up of three poles, one is a magnetized part (S pole) 27 which is formed by arbitrarily extending one of a plurality of the S pole parts constituting the permanent magnet 22 in the axial direction of the rotor 15, in such a way as to face the Hall element and the other two are a pair of magnetized parts (N poles) 28, 29, which are formed by similarly extending two N pole parts neighboring each side of the S pole part constituting the one magnetized part 27.