PURPOSE: To reduce the uneven wall thickness and produce less inner surface flaws during the inclined drawing and rolling operation.
CONSTITUTION: The length Lt of the part where a plug is brought into contact with the material is set 6π.D1.SIN(F). η/N or more. The length Lr of the reeling part of the plug is set π.D1.SIN(F). η/N or more. The relationship between the effective surface angle αR of the roll and the surface angle αp of the reeling part is αR≤αp≤[αR+0.25(deg) ]. The uneven wall thickness is reduced thereby. Where, N is the number of the roll, η is the advancement efficiency, D1 is the diameter of the material after rolling. The position where the material starts contact with the plug is set rearward of the roll gouge and the distance from the roll gouge to this starting position is set π.D1.SIN(F). η/(N.e) or less. This arrangement reduces inner surface flaws, where (e) is the stretching ratio.