PURPOSE: To improve yields in punching and to simplify assembly by punching a lateral T-shaped central iron core plate and a laterally-widened longitudial T- shaped side iron core plate.

CONSTITUTION: A pair of lateral T-shaped central iron core plates 1 are punched out from the right and left portions of a rectangular magnetic path material plate 2 in such a way that the length of a logitudinal arm 1a of the central iron care plate 1 is made equal to that of a longitudial side of the magnetic path material plate 2, the arm 1a is made situated at the right and left ends of the material plate 2, and the end of a lateral arm 1b is made positioned in the vicinity of the center of the material plate 2. Then, after punching out at the middle of the central longitudial arm of the remaining I-shaped portion of the material plate 2, a pair of laterally-widened longitudial T-shaped side iron core plates 3, each comprising a lateral arm 3a and a short longitudial arm 3b, are punched out.