PURPOSE: To improve touch feeling on a fingertip and to increase the strength of a holding part by forming the holding part in hollow cylindrical shape by forming a finger hanging recessed part between a side wall.

CONSTITUTION: The holding part 5 is formed in a hollow cylinder 7 by forming the finger hanging recessed part 5a between the side wall 3 at a confronting side wall outer part at the short side of a ball box 1. When such formation is performed, part of the hollow cylinder 7 is formed successively at an overhanging edge 4 when the ball box 1 is molded, and a remaining part prepared separately is coupled with it integrally. The outer plane of the hollow cylinder 7 is formed in a curved surface to obtain satisfactory touch feeling on the fingertip.