PURPOSE: To equalize the wall thickness throughout the entire pipe length and to improve the finishing of the pipe inside surface by moving a plug in the inverse direction to the pipe carrying direction and narrowing a gap between the plug and grooved rolls in proportion to a moving amount of the plug.

CONSTITUTION: A load cell 7 detects a pipe stock 1 and sends detection signals to an arithmetic and control unit 8 when the stock 1 is supplied between grooves rolls 2a, 2b and a plug 3. A relationship between a plug moving distance after biting and an axis-to-axis distance of the rolls 2a, 2b being changed in propor tion to the plug moving distance is previously set in the unit 8, which changes a pressure oil supply amount to a hydraulic cylinder 6 to reduce the axis-to-axis distance of the rolls 2a and 2b when a pipe stock carrying distance after biting equals to a prescribed distance. In that method, both the dimensional accuracy and quality of the tube inside surface are improved.