PURPOSE: To prevent the generation of spiral marks and the wear of a plug and to obtain a product having excellent inside surface quality by using a roll having a circular conical shape on one side, the roll inlet side of which is formed to a linear shape without having a face angle and the outlet side is formed to the same face angle as the taper angle of the plug.

CONSTITUTION: The roll 1 is formed to the linear shape (cylindrical shape) having 0 face angle with the roll gorge part 1a as a boundary on the inlet side to which a stock pipe A advances. The outlet side bordering the roll gorge part 1a is made into the circular conical shape formed to have the same face angle as the taper angle of the plug 2. The roll inlet side is thus formed to a straight line without having the face angle, by which the abrupt rolling down in the roll gorge part is suppressed. The face angle at the roll outlet is made coincident with the face angle of the plug, by which the generation of the spiral marks is prevented.