PURPOSE: To make the wall thickness size of a seamless tube constant by changing simultaneously the inclined roll opening and the plug advanced quantity and executing the rolling, in a pierce rolling process by a piercing mill.

CONSTITUTION: The roll opening being a roll interval of inclined rolls 1l, 1r is adjusted by a first automatic positioning device 61, a thrust block for regulating a movement of a mandrel bar M for moving a plug 2 in its axial length direction is controlled by a second automatic positioning device 62, and the plug advanced quantity is adjusted. By changing simultaneously this inclined roll opening and the advanced quantity, and making a tip draft rate of an intermediate part of a material to be rolled smaller than that of its top part and bottom part, a flaw on the inside surface of the material to be rolled is suppressed and the wall thickness size after pierce rolling can be made constant.