PURPOSE: To facilitate the setting work when an insert-molding operation is conducted by a method wherein a flat wire coil, which is formed by welding or soldering a lead wire, is formed into a molded body by insert-molding using the resin containing ferrite powder.

CONSTITUTION: A flat type wire 2, which is formed by welding or soldering a lead wire, is insert-molded by the resin containing ferrite powder, and a molded body is formed. As a result, the rigidity of a coil is increased, and the setting operation is facilitated when the coil is insert-molded. Also, after the flat type wire 2, which is formed by welding or soldering a lead wire, has been insert-molded using the resin containing ferrite powder, the center part in thickness direction of the molded body is cut on the plane surface at right angle to thickness direction, and it is divided into two parts. Accordingly, as the coil is positioned on the same surface as the opposite surface of a rotary transformer, the coefficient of coupling is brought into the best condition. Also, as a flat type wire is used, the cross-section area can be made larger than the case where a round wire is used.