PURPOSE: To enable a casing process to be easy and improve productivity by impregnating or coating a synthetic resin to a magnetic core where a magnetic material is laminated, inserting a spacer into a slit which is formed by cutting one part of the magnetic core, and then storing it into a mold and then performing injection and filling of a molten synthetic resin.

CONSTITUTION: A magnetic core 2 is formed by winding a thin amorphous metal film 1 in ring shape. Then, after performing vacuum dipping of an epoxy resin to the magnetic core 2, it is cured. After making a slit 6 to the magnetic core 2, a spacer 8 is inserted into it, this magnetic core 2 is placed into a mold for injection molding, and then a molten synthetic resin is injected and filled into the mold, thus enabling the surrounding of the magnetic core 2 to be cov ered with a synthetic resin layer 5 as a casing. No slit 6 is formed at the mag netic core 2 when making a complete closed magnetic circuit and other parts are performed similarly by the above manufacturing method. A resin to be used should be selected according to the performance requested to a core and especially nylon, polyethylene, polyacetal, and a mixture of a reinforced material such as fiber glass to them are suitable.