To improve the accuracy of inside diameter at tube ends in the manufacturing method of a seamless tube by Mannesmann plug mill process by increasing thickness in both end parts of the tube by controlling the opening degree of roll and inserting a plug having a prescribed outside diameter into the tube end part where the inside diameter is reduced.
At the time of manufacturing the seamless tube by Mannesmann plug mill process, the opening degree of roll is controlled so as to increase thicknesses at both tube ends of the seamless tube 2 and the outside diameter of the seamless tube 2 is equal in the axial direction but the wall-thickness is thick in both end parts of tube and the inside diameters at the tube ends become smaller than a prescribed inside diameter. When the plug 3 having the outside diameter equivalent to the prescribed inside diameter is inserted into the tube end part where the inside diameter is reduced than the prescribed inside diameter under a hot or a cold condition, the tube end part is restrained on the side of the inside diameter and the accuracy of inside diameter is improved.
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