PURPOSE: To drastically improve the nose bend of the tip of a seamless tube by inserting a mandrel into the inner part to perform sizing in passing a sizer, in the sizing executed after passing a reeler in the manufacture of the seamless tube by Mannesmann plug mill method.

CONSTITUTION: Since, in time of manufacture of a seamless tube and in the sizing executed after passing a reeler, the mandrel 1 is inserted into the inner part of the tube in passing the sizer and the sizing is executed, the tube is restrained by the mandrel from the inner part and guided. For this purpose, even when the timing corresponding to rolls facing the sizer is not normal or a small difference between circumferential speeds changes reaction in the axial direction, buckling does not occur and nose bend is prevented from generation. Insertion of the mandrel 1 into the inner part of the tube in the sizer is performed from the side of the final stand, for example, until the tip of the mandrel comes to the 2nd stand 5 and the 1st stand 4 before the final stand. After the tip of tube, 2-3 meters, is rolled, the mandrel is moved back to sizing.