PURPOSE: To improve the accuracy in shape and to improve productivity with a reduced clamping time by clamping an object formed to a cylindrical shape to a mandrel by means of right and left clamps, and clamping the intermediate part by means of a leaf chain having rollers, and providing a device for matching the end faces of the cylinder.

CONSTITUTION: A cylindrical object 10 is inserted into a mandrel 12 by regulating the joint thereof with a position regulating metal 11. A cylinder 14 is then actuated to fix the object 10 to the mandrel 12 by means of a left clamping metal 13, then the metal 11 retreats. A right clamping metal 15 is actuated by a small diameter cylinder 17 to clamp the object and at the same time, the pressure of the cylinder 14 is evacuated. End matching cylinders 19, 20 advance to correct the deviation between the right and left end faces. The object is again clamped by the cylinder 14 and in succession, a chain 21 is wound on the mandrel 12 by a chain clamp 23, by which the object is clamped. A large diameter cylinder 18 is finally actuated to clamp the object by the metal 15, then the object is subjected to mushroom welding.