PURPOSE: To prevent generation of bends in a tube by passing a seamless tube into a sizing stage mill in twice, by bringing the top end in a tube polishing stage to be the top end in a 1st passage, and by bringing the rear end in the polishing stage to be the top in a 2nd passage.

CONSTITUTION: A round billet 1 heated by a heating furnace 2 is pierced by a piercer 3 and is thickness rolled by a plug mill 5, and then a produced tube is polished by a reeler 6. Next, after sizing a tube 10 by a 1st sizer 7, the tube 10 is turned to reverse the top end to be the rear end and is sized by a 2nd sizer 8 again. The tube 10 on the outlet side of the reeler 6 has respective bends in the top and rear end parts and the rear end bend is eliminated in sizing by applying a tension to the tube 10 by the 1st sizer. Further, the other end bend is also eliminated by passage through the 2nd sizer in on-line. Hence, generation of bends in intermediate working stages is positively eliminated, so that generation of tube bends is eventually prevented.