When the magnet holding spring is in the deployed state of the plate-like shape, the auxiliary pole is beforehand fixed to one of the side-plates, then the magnet holding spring is bent into a “]”-like sectional shape. As a result, the magnet holding spring and the auxiliary pole that are assembled integrally constitute the sub-assy. Convex parts are formed on the auxiliary pole. The auxiliary pole is fixed to one of the side-plates by press fitting the convex parts to the fitting holes formed on one of the side-plates. The auxiliary pole is laid on one of the side-plates in the direction of a plate thickness for positioning before forming the convex parts on the auxiliary pole. In the positioned state here, by embossing the auxiliary pole, the convex parts are formed on the auxiliary pole, and at the same time, the convex parts are press fit into the fitting holes thus the auxiliary pole is fixed to one of the side-plates.