PURPOSE: To prevent abnormal thickness and to reduce dispersion in length or thickness of tube stocks by learning and controlling the amount of advance of a piercing plug basing on the actual thickness or correction term when rolling the next material at each plug bar.

CONSTITUTION: The density of a hollow 18 is calculated from estimated temperature of the hollow 18 in an exit side of a piercer 11 and the targer length of the hollow is calculated from the weight of a billet measured beforehand by a billet weight measuring device 30 from instructed hollow thickness, instructed hollow outer diameter. Then, the actual hollow thickness is determined using the actually measured hollow length lj measured by a hollow length measuring device 32 and the actual hollow outer diameter Dj measured by a hollow outer diameter measuring device 34. Then, the actual hollow thickness correction term is calculated and stored. When rolling the next material, learned controlsetting goege and lead are calculated from the actual hollow thickness correction term. After changing plug diameter, first rolling is completed, and when rolling of third time and later, the optimum learned control setting gorge and lead are set at each plug bar 16, and learned control is performed.