PURPOSE: To stabilize an abutting state by providing a relief stepped part to prevent a pole contact surface in a non-shading part from contacting a pole contact surface on an opposite side at the outer edge of a pole contact surface at least on either of a fixed iron core and a movable iron core.

CONSTITUTION: In an AC magnet iron core apparatus consisting of a fixed iron core 14 fitted with an operating coil and a movable iron core 20 with a shading coil 35, the fixed iron core 14 having a small step h1 formed in the internal side of a pole contact surface 38 and a large step h2 in the external side, is provided. After cooling, the pole contact surface 38 tilts low inwardly because of signification thermal expansion in the pole contact surface caused by the insufficient release of heat generated during polishing. In order to make sure that a pole contact surface 38b in the shading part of the movable iron core 20 comes in contact securely with the pole contact surface 38 of the fixed iron core 14, a step difference Δh is formed in such a manner that the pole cntact surface of an external non-shading part 40a may be slightly lower than the pole contact surface of a shading part 42.