To solve such problems that in a method of packing a setscrew separately as an accessory, a part may be lost and setting thereof is troublesome; and the end of the setscrew sticks or an injury may be caused when the screw supporting structure described in the Japanese Open Utility Model Bulletin (1988-57810) is employed.

A screw housing part 9 comprises a protecting wall 6f for protecting a screw end of the setscrew 9c, a stopper 9b coming in contact with the head of the setscrew 9c, and holding parts 9a symmetrically opposed with an axis of the setscrew 9c as a line of symmetry and arranged sandwiching the shaft part of the setscrew 9c and formed shorter than the shaft part of the setscrew 9c. The head part of the setscrew 9c is elastically grasped between the ends of the stopper 9b and the holding part 9a. The head part of the setscrew 9c is formed to gradually rise from the holding part 9a with making the other end of the holding part 9a be a fulcrum by pressing down the shaft part of the screw end side of the setscrew 9c.