To minimize the recess of a material, and prevent various flaws by providing a fixing means for fixing each piece of a guide bush aperture divided by slitting.

A regulating nut 20 is fastened with confirmation to the degree to which a material W is fixed in the point of a distance (h) (about 30mm) from the aperture top of a guide bush 101. After this regulation, three fixing screws 103 are fastened in order little by little, and uniformly fastened with discretion until the material W is smoothly penetrated. By this regulation, the material W becomes smoothly movable to the left, and each cut piece 101a, 101b, 101c of the guide bush 101 are mutually fixed.