PURPOSE: To prevent the leakage of a line of magnetic force to the outside by attaching and fixing magnetic shielding plates to the upper and lower surfaces of a magent and generating an adsorbing effect by the adhesion and mounting of a male stud to a magnet surface.

CONSTITUTION: The ferromagnetic plate 2 is brought into contact with one side of the magnetic pole surfaces of the permanent magnet 1 and the ferromagnetic plate 3 with the other magnetic pole surface, and both plates are fixed and set up integrally by a case consisting of a nonmagnetic blank while a projection 2a up to the upper end of a hole 1a is erected to the plate 2. The case 4 takes a cylindrical pan shape, a hole communicated with the hole 1a is formed to the inner bottom of the case, a hole collar 4b connecting being in contact with the inner circumferential edge of a hole in the plate 3 is formed and a pawl hole 4a is formed to an opening edge, the plate 3, the magnet 1 and the plate 2 encased in the case 4 are fixed and set up integrally by a turn-in to the surface of the plate 2 of said pawl piece 4a, and a cylinder body 13 made of synthetic resin is fitted tightly.