To facilitate fixing of a square core by a method wherein a space connected to a square hole is formed in a boss part, and a stopper to support the square core, inserted in the square hole, in a state to protrude in the direction of a deep part is oscillatorily contained in the space.

In a state that a square core 1 is not inserted in a square hole 11, a protrusion piece 24 of a stopper 20 is pushed up through the force of a spring 30 and teeth 23 formed on a rotary disc 21 are brought into a state to protruded in the square hole 11. Namely, an arcuate surface on which the teeth 23 are formed is inclined based on the central line of the square hole 11 and the teeth 23 positioned on the deep side of the square hole 11 are protruded in the square hole 11. By inserting the square core 1 in the square hole 11, the teeth 23, protruding in the square hole 11, of the rotary disc 21 are pushed up and the rotary disc 21 is rotatable counterclockwise against the compression force of the spring 30, whereby the square core 1 is inserted in the direction of the deep part of the square hole 11. The square core 1, since the teeth 23 of the rotary disc 21 are brought into a state to effect protruding support of the square core 1 in the direction of the deep part of the square hole 11, is incapable of being pulled out from the square hole 11.