To make it possible to install a plurality of permanent magnets accurately on an inner face of a yoke by correctively processing a cylindrical material and thereby further increasing a roundness and a dimensional accuracy.

With a spring back of a cylindrical material 3 taken into consideration, a radius of curvature of an outer face of eight diameter-expanding members 5 is made larger than that of an outer face of permanent magnets 8. Furthermore, the outer faces of the diameter-expanding members 5 are pushed in the radius direction outside of an inner face of a yoke 2 by means of a pressing member 6. At that time, an outer face of the cylindrical member 3 is bound by an inner face of an outer face binding member 7 and so, the substance of the cylindrical material 3 does not spring outside in the radius direction and the cylindrical member 3 is corrected to a cylindrical shape. Therefore, a radius of curvature of an inner diameter of the cylindrical material 3 becomes larger than that of an inner diameter of the yoke 2 when the diameter of the cylindrical material 3 is expanded. After the expansion of the diameter, however, the radius of curvature of the inner diameter of the cylindrical material 3 becomes nearly equal to that of the inner diameter of the yoke 2.