PURPOSE: To achieve simplification of a structure around a clamp pad by providing at least two rod-like members capable of being freely extended and contracted in the axial direction and a stopper length adjusting means for changing the axial length of one side rod-like member on a pad stopper for regulating the turning range of the clamp pad.

CONSTITUTION: A rod 2 is inserted from the top side of a cylindrical body 1, and a screw part 2b on the tip is screwed into a threaded hole of a nut supporting part 4a and fixed in a state where the projecting member is covered with a compression coil spring 3. A square hole for a hexagon wrench is formed on the end surface of a flange part 2a of the rod 2 so that screwing may be performed by using the hexagon wrench. Further, a top plate 6 is screwed and fixed on the top side of the cylindrical body 1, a screw member 5 is screwed to the nut 4b, therefore, a pad stopper 26 can be assembled. This pad stopper 26 is previously assembled in the other process and assembled to a clamp arm 22 and a clamp pad 24 by pins 15, 17.