PURPOSE: To conform the ratio between the length of a blank material and a pipe material ao a desired value, by automatically controlling a roll gap and a guide-shoe gap by operating from respective measured values of the weight and length of a blank material and a pipe material after piercing.

CONSTITUTION: In piercing a material by a Mannesmann piercer, supporting the outer circumference of the material by rolls 1 and guide shoes 2 and piercing with a plug 4 supported by a mandrel 5, a desired piercing hole-length l is obtained by an arithmetic unit from the hole piercing ratio, given by a setting unit 14, to the weight of blank material measured by a weigher 12; and average sectional area is obtained successively. Next, a roll gap S1 of the rolls 1 and a gap S2 of the guide shoes 2 are obtained from said values to be outputted to respective drive-controlling units so as to make them perform automatic positioning-operations. The correcting actions for piercing the next blank material is performed by obtaining the correcting values of S1 and S2 from l, l0 and the weight of material, after detecting the length l0 of the pipe material which passed through the piercer 11 by a detecting unit 13.