PURPOSE: To prevent flaws of a magnetic pole surface and an attracting surface, by bonding a protecting film to both or either of the magnetic pole surface of a permanent magnet and/or the attracting surface of a ferromagnetic plate.

CONSTITUTION: A protecting film 7 is bonded to both or either of a magnetic pole surface (b) of a permanent magnet and/or the attracting surface of a ferromagnetic plate 3. At the time of assembling and at the time of attaching, bent leg pieces of stoppers, the corner parts of a case and other protruding parts hit the magnetic pole surface (b) or the outer surface of the case on the side, where a male tool is attracted in the case, when the permanent magnet is assembled in a female tool in the case. Since the protecting film 7 is bonded on said plated surface, however, the flaw of the plate surface is prevented. When the attachment is completed, the protecting film 7 is released, and the fine surface of the plated film can be maintained.