PURPOSE: To improve the product quality by setting both face angles of the expansion part and the sizing part of a roll within respective specific angle ranges and installing the reeling part of a plug to face to the roll sizing part.

CONSTITUTION: For a roll 1 having a circular truncated cone contraction part 1a, expansion part 1b, and sizing part 1c, a roll face angle α2 of the expansion part 1b is set so that α2>5°, a roll face angle α3 of the sizing part 1c is set so that α3<α2 and 0°≤α3≤10°. Further, a reeling part 3b of a plug 3 is put to face to the sizing part 1c of the roll 1 and a face angle of a reeling part 3c is set to nearly equal to the roll face angle α2. Thus, a propelling force against a stock to be rolled is increased and wall thickness deviations are restrained as well as choking is prevented, so that the product quality is improved.