To reduce back pressure applied to a mandrel at the time of hot extrusion, to stabilize the quality and to reduce the manufacturing cost of the mandrel by making the step diameter of the mandrel equal to the diameter of the parallel part and taking the clearance between the stepless part and the hole of a pressing disk larger.

The step diameter is equalized with that of the parallel part 4. To manufacture the mandrel 3, a peeling base material having the diameter which is the diameter of the parallel part plus 1 mm is finished into the diameter of the parallel part 4 and a screw is cut in the rear end part. It is preferable to prepare the bore diameter of the pressing disk 2 so that the clearance of 0.5-1.5 mm is formed between the hole and the step diameter and to use it as an exclusive pressing disk. The clearance between the hole of the pressing disk and the step diameter is enlarged, the automatic alignment of the tip is preferentially executed and the step diameter is made equal to the diameter of the parallel part 4, thus the back pressure applied to the mandrel 3 is reduced.