PURPOSE: To reduce a wall thickness deviation of pipe and to prevent a pipe from streak flaws generating in its inner wall surface, by actuating tensile force in the axial direction of a plug bar during rolling and rolling a material by gradually moving a plug in its axial direction.

CONSTITUTION: A plug bar 19 is inserted to a hollow 30 by a plug bar inserter 16 at the position of an intermediate roller table 14. The hollow 30 is transferred to the position of a roller table 20 located at the inlet side of a plug mill, and the rear end of the bar 19 is chucked by a bar holder 22, and then the holder 22 is advanced and stopped. Next a pusher 24 is advanced to force the hollow 30 into rolls in order to roll the hollow 30. The plug 18 and the plug bar 19 are gradually retreated in a direction (arrow mark B) opposite to the rolling direction. Accordingly, a force acting on the plug 18 and thermal load are reduced without rolling the hollow 30 at the fixed position. After completing the rolling, the holder 22 is retreated to the fixed position, and the chucking is released, and then the plug bar 19 and the plug 18 are cooled to be reused by circulating them.