To prevent buckling of a rivet for fixing a cover by decreasing the hole diameter positioned more inwards than a section where a cylindrical cover fitting part in a through hole is fitted for the formation of step-off in the through hole.

A through hole 4 is formed by stacking several core sheets 2 having a through hole A whose hole diameter is H so as to obtain stacking thickness C more than the length B of a cylindrical fitting part 6 having the outside diameter H of a cover 5, stacking several core sheets 3 having a through hole whose hole diameter is E and which is larger than the shaft diameter D of a rivet T and is smaller than the hole diameter A on the core sheets 2 so as to obtain stacking thickness G more than the length B of a cylindrical fitting part 6 of the cover 5. The hole diameter E of the through hole 4 is not restricted by the cylindrical fitting part 6 having the outside diameter H of a cover 5, therefore, it is possible to decrease a clearance between the rivet 7 and the through hole 4. As a result, the rivet 7 will not buckle inside the through hole 4.