PURPOSE: To reduce a tension acting on a shell and to improve the stripping capability by evaluating stripping capability of a mandrel bar during rolling by a mandrel mill and controlling a tension between the mandrel mill and a sizer based on the evaluated stripping capability.

CONSTITUTION: A tension after finish of rolling by a mandrel mill is estimated by measuring a tension before finish of the rolling. Therefore, an allowable limit of a tension acting on a shell 3 before finish of rolling by a mandrel mill is set and an unusual situation is decided when a tension exceeds the limit. Thus, the stripping capability of a mandrel bar 4 is improved by controlling rolling speeds of a mandrel mill 1 or a sizer 2 to bring a tension acting on the shell 3 not to exceed the allowable limit when an unusual situation is recognized during rolling by the mill 1.