PURPOSE: To enable the prolongation in lifetime of a core by relieving the stress acting on fixed pins, by a method wherein an organic film is formed at part or over the whole of the inside surface of the side plate or at least one surface of an Si steel plate.

CONSTITUTION: A core 1 for electromagnet has a core 2 provided with side plates 3, 3 on both sides, which plates are caulked with five fixed pins 4, 4...4. The core 2 is constructed by laminating Si steel plates 5, 5...5 punched in E form. An organic film 6 is formed to each Si steel plate 5 by deposition in lattice form. This organic film 6 can be formed by deposition either to both surfaces of the Si steep plate 5 or to the inside surface of said side plate 3.