PURPOSE: To improve the quality of a pipe by calculating the number of revolution of a billet from the position where the billet is bitten by the cross helical rolls up to the top end of a plug and correcting and controlling the angle of inclination, etc. of cross helical rolls to make the calculated values coincident with the target value.

CONSTITUTION: An arithmetic control part 25 reads the outside diameter and length of a blank pipe 5 from an outside diameter gage 21 and a length gage 22 and corrects the correction rates of the cross helical roll gap ang plud lead in order to make the outside diameter and length coincident with perliminarily determined target values. The rolling conditions for the succeeding piercing are calculated from the correction rates. The control part calculates the number of revolution of the billet from the position where the next billet is bitten between the cross helical rolls 1l and 1r until the billet arrives at the top end of the plug 2 in accordance with the calculated values. The control part corrects the angle of inclination or the angle of intersection of the rolls 1l, 1r to make the calculated values coincident with the prescribed target values and outputs the same to automatic setters 28, 29. The flaws on the inside surface of the pipe 5 are decreased and the quality of the pipe is improved by the above-mentioned method.