PURPOSE: To automatically command plug changes in piercer mills piercing tube stocks by calculating a plug diameter of a piercer mill by measuring the weight of the tube stock being to be pierced and the outer diameter and length of the pierced tube stock and outputting a plug change command when the above calculated value is smaller than the change criterion.

CONSTITUTION: A measuring instrument 60 measures the weight W of a tube stock 10 before the stock 10 is pierced by piercer mills 40W50. The length L and outer diameter D after piercing are measured by measuring instruments 70 and 80 after the stock 10 is pierced. Then, a plug change commanding device 90 calculates a plug diameter of piercer mills 40W50 based on the measured weight W, length L, outer diameter D, and an information inputting device 140. The device 90 decides which of changing or not changing based on the calculated plug diameter and the criterion value and commands a plug change command.