PURPOSE: To simplify a manufacturing process, and to mass-produce the title cores by impregnating the inside of a core case with a resin under the state in which a roll body on which a magnetic material sheet is wound is arranged into the core case.

CONSTITUTION: A roll body 1 in which a magnetic material sheet 10 consisting of an amorphous magnetic material, permalloy, a silica steel plate, etc. is wound and an end section is fixed through spot welding, etc. is prepared, and placed into a core case proper 12 made of an insulating resin. Varnish or an insulating resin such as epoxy is casted into the core case proper 12, the roll body 11 is impregnated with said resin, and the roll body is covered with a core case cover body 14 made of the insulating resin after an impregnation with the resin. The core case cover body 14 is fitted, the roll body 11 is cut together with the core case to form a gap 15, and a nonmagnetic spacer 16 is inserted into the gap 15, and fixed with a resin, etc. Accordingly, a manufacturing process is simplified, thus improving mass productivity.