To firmly fix a square core inserted in a square hole by a method wherein an elastic member is disposed between the square hole and the square core and the square core is pressed from two directions through the force of the elastic member.

Elastic members 10 are arranged on the horizontal surface and the vertical surface of the square hole 41 such that a square hole 41 is made narrower in the two directions of a horizontal direction and a vertical direction. When a square core 4 is inserted in the square hole 41 in which the elastic members 10 are arranged, a force to cause movement of the elastic members 10 in the advancing direction of the square core 4 is applied. However, since a lock piece 11 is brought into abutment against the end face of a hub 40, the elastic members 10 are prevented from displacement in a position. The square core 4 inserted in the square hole 41 deforms the arcuate elastic member 10 into a flat state and the square 4 is pressed in a direction in which no elastic member 10 is arranged. As a result, the square core 4 is pressed in a vertical direction and a horizontal direction and the square core 4 and a hub 40 firmly fixed both in a vertical direction and a horizontal direction.