PURPOSE: To drastically improve the nose bend at the tip of a seamless tube by opening the roll gap more largely than a set value in advance when the tip of a tube stock passes and closing the roll gap down to the set value for sizing directly after the tip of the tube stock passes the center between roll axes.

CONSTITUTION: In the sizing by a sizer executed after the tube stock passes a reeler, the gap between the rolls 2 is opened in advance more largely than the set value when the tip of the tube stock passes, and it is reduced to the set value for sizing directly after the tip of the tube stock passes the center between the roll axes. Each roll, when the tip of the tube stock passes it, is opened so that the tube stock has a hole diameter larger than the outer diameter of the tube stock rolled at the preceding stand. the tip of the tube stock passes the center between the roll axes without coming in contact with the rolls, therefore, buckling is not generated and a nose bend is prevented from generation. In this way, since the nose bind is improved drastically and a scrapping part is limited to a non-sizing rolled part at the tip, the scrapping part is reduced drastically and the yield is improved so much.