PURPOSE: To suppress eccentricity of the center of rotation according to a change of temperature by fixing a bush member press-fitted to an internal peripheral surface of a yoke member in an intermediate part in an axial direction so as to improve fitting of the yoke member to a hub member.

CONSTITUTION: In a free end part of a large diametric part 22a of a yoke member 22, a large internal diametric part 22c of a little larger internal diameter is provided. The external diameter of an upper part 30b above a stepped part provided in almost the central part in an axial direction on the peripheral surface of a bush member 30 is a little smaller than the external diameter of a lower part 30c in a lower side. When the bush member 30 is thus connected to the yoke member 22, the fixing region of both the members is relatively short in the axial direction, and a slight space between both the members at both end parts of the bush member 30 is created. By providing this space, eccentricity of the center of rotation of a rotor hub can be effectively prevented even when the ambient temperature is changed.