A magnetic core portion 16, a residual base material portion 42, and a connecting portion 43 are formed by pressing from a base material 40. Notches 18A are formed at a peripheral edge of the magnetic core portion 16, which is bent in the direction of the plate thickness to form a temporary fixing portion 18. When base materials 40 are overlapped to cut the magnetic core portions 16 from the connecting portion 43, the magnetic core portions 16 are pressed in directions toward one another so that the temporary fixing portion 18 is filled between notches 18A in the other magnetic core portion 16, to fix temporarily the other magnetic core portions 16. Since the magnetic core portions 16 are temporarily and simultaneously fixed by pressing in a cutting step normally carried out, the magnetic core portions can be temporarily fixed with ease.