PURPOSE: To easily and accurately predict pullout capability by judging insufficient pullout capability of a mandrel bar at the time that an electric current value of a driving motor for rolling rolls of a sizer exceeds a limit value when a mandrel mill and the sizer are concurrrently operated.

CONSTITUTION: Emergency actions are taken by making judgment on insufficient pullout capability of a mandrel bar 4 pulled after finish of rolling by a mandrel mill 1 in case an electric current value Ip of a driving motor 9 for rolling rolls of a sizer 2 exceeds a value of Ipm when the sizer 2 and mandrel mill 1 are concurrently operate din a restrained mandrel mill composed of the straight lined sizer 2 and mill 1. The Ipm is determined based on experiments and actual operations. After finish of rolling by the mill 1, the bar 4 is pulled out a shell 3 by a bar constrictor 5.