PURPOSE: To improve the strength of tapered parts, enable effective formation and mass production by forming a head part where leg pieces each having a tapered part at its foremost end while being connected to each other at the base, i.e., the rear end, thereof mutually closed, in a flat head shape projecting from a cross sectional flat line in the cross direction by caulking.

CONSTITUTION: Two leg pieces 2 which are provided with tapered parts 1 formed at their one end parts and closed mutually at the other end parts are formed of a cross sectional flat wire material, and a head part 3 connecting both of the leg pieces 2 to each other at the base in a flat head shape projecting in the cross direction by caulking. The structure becomes simple to thereby enable mass production at low cost accordingly.