A wound wire is wound around a core assembly so that both ends are short-circuited. In a coupling pin insertion state in which a coupling pin is inserted in a through hole of the core assembly, outer-peripheral space parts of respective divided core portions of the core assembly are disposed so as to overlap in plan view. Consequently, an air gap is formed in a part of a side surface of the core assembly. Before formation of a covering member, a main wire is caused to pass through the air gap and is thus disposed in a wiring hole of the core assembly. Then, the covering member for closing the air gap is provided on an outer peripheral surface side of the core assembly including the air gap so that a core structure is obtained.