PURPOSE: To stably operate a mill and to improve yield by stably executing bar stripping of a mandrel mill.

CONSTITUTION: The bar retained force in rolling is measured at only the final stand of the mandrel mill 4 and also 3μp(SB--SF)+FF-G (Where, μ: the coefficient of friction, p: rolling pressure, SB: backward slip region, SF: forward slip region, FF: front tension, G: the component of force in the axial direction of the rolling pressure.) at the point of time of completion of mandrel rolling is calculated. When this calculated value is larger than the bar retained force, rolling is continued as it is. When the calculated value is equal to or smaller than the bar retained force, a pressure necessary to bar stripping is determined, the roll gap of an extractor 5 is reduced till the bar retained force becomes a specified value and also the correction of revolving speed attendant on this change in roll gap is executed.