PURPOSE: To prevent the generation of the interior defect of tube by specifying piercing rolling conditions at the time of manufacturing a seamless tube.

CONSTITUTION: At the time of manufacturing the seamless tube, when a billet (base stock) 2 is piercing-rolled with a piercer, the interval between rolls 1, 1 is made smaller than the set value by 1-4m and the tip draft rate of a plug 3 is made higher than the set value until the start of piercing and, at the same time of the start of piercing, the roll interval is expanded and the tip draft rate of the plug is decreased to 3.0-4.5%. In this way, even if it is a base stock made by rotatary continuous casting method, the generation of interior flaw when piercing rolling can be drastically reduced. Then, it has the great effect of contributing to the cost rationalization of the billet for manufacturing seamless tube.