A hole edge of a recessed section (18) has a shape of parallelogram, and is composed of a pair of first hole edges (19, 19) parallel to a first direction and a pair of second hole edges (20, 20) parallel to a second direction. The first hole edges (19) of respective recessed sections (18) in the first direction are arranged on a straight line along the first direction, and the second hole edges (20) of respective recessed sections (18) arranged in the second direction are arranged on a straight line along the second direction. The recessed section (18) is formed by pressing a wire barrel (16) by using a die (24) whereupon a plurality of protruding sections (25) are formed at positions which correspond to the recessed sections (18).