PURPOSE: To prevent the generation of an eccentric wall-thickness deviation and to obtain a high-quality seamless pipe by connecting swingably a plug to a mandrel bar and aligning the axial center line of the plug with that of a material to be rolled.
CONSTITUTION: In the 1st piercer A, a billet B is moved by biting it between rolling rolls 1l, 1r, to pierce it into a hollow shell H by a plug 2. Next, in the 2nd piercer A', the shell H is spirally moved by the rolling rolls 1l, 1r, to expand it by a plug 2a. A connecting member 31a is formed at the tip of a mandrel bar 3a used for supporting the plug 2a, and the tip surface of member 31a is processed into a spherical convex surface 24. A connecting boss 32a to be loose fitted to a recessed connecting hole 25 of plug 2a is provided to the center of the surface 24, to make the plug 2a swingable with respect to the bar 3a. In this way, the axial center line of plug 2a follows up the pass-center of hollow shell H, to prevent the generation of an eccentric wall-thickness deviation.