PURPOSE: To prevent the increase of electric power for driving rolling rolls during piercing of the rear end of a round billet by heating the rolling rear end of the round billet and piercing the same in the stage of piercing the round billet with a cross helical rolling mill and producing a seamless steel pipe such as high alloy steel pipe or large diameter steel pipe.

CONSTITUTION: A bullet-shaped plug 20 is inserted to the sectional center of the round high-alloy steel billet or large-diameter round steel billet of a high temp. taken out of a heating furnace and while the plug 20 inserted part of a pipe stock P which is made hollow by such insertion is spirally rotated and advanced by the rolling rolls 21, 21, the central part of the billet 1 is pierced. The electric power of the motor for driving the rolls 21, 21 sharply increased when a holes in penetrated by the piercing at the final end of the billet 1. The final rear end of the round billet is preliminarily and separately heated to maintain said end at the increased temp. in order to prevent such increase. The increase of the electric power when the hollow hole is penetrated is thus prevented.