PURPOSE: To reduce leakage flux and to improve economy and machining efficiency by forming axial groove sections, with same interval, in the circumferential direction, in at least one of inside or outside pole structure to be arranged on a coupling main body by means of a gear cutting machine.

CONSTITUTION: Recessed groove sections are formed, by means of a gear cutting machine, in the outer circumferential face of an inside pole structure 1a and the inner circumferential face of an outside pole structure 1b facing therewith. Radial thickness of the annular pole structures 1a, 1b is set as required actually, then the inner and outer circumferential sides thereof are cut in to form groove sections 3c, 3d having tooth-type cross section entirely in the axial direction. Since excessive cutting work at iron portion is not required, machining process is shortened thus realizing high saving effect of resource and energy. Since the gear cutting machine has higher machining accuracy than drilling machine, fluctuation of pole width W is suppressed and the cutting depth H can be set to a random value.