PURPOSE: To completely achieve a support of a rivet head by forming a shallow countersink at an upper of a countersink section and providing a wider supporting area for the rivet head.

CONSTITUTION: A part of a surface 40a forms a second frustum section of an opening 28, which is heading inward and is gradient downward at about 10 degrees. An area of a shallow countersink shape is formed at an upper of the countersink section or a first frustum section 32 thereby. A large rivet head is further formed at the upper of the counter section or the first frustum section 32. Diameter of a cylinder section 34 and inward gradient section 40 are larger than the frustum section, and have a wider support area to achieve a perfect support of the rivet head.