PURPOSE: To attain a stable operation, and to improve quality by controlling shell tension between stands in a posterior stage according to a deviation between calculated shell tension and target shell tension.

CONSTITUTION: The adjusters 8 of the number of rotations and load cells 9 are mounted on the plural stands A1 to An of a retract mandrel mill. The current and the number of rotations of a motor 10 for driving mills of the stands A1 to An, are measured, and are transmitted to an arithmetic and control unit 11. The load torque of the motor 10 of a mandrel mill 1 is estimated according to the current and the number of rotations of the motor. Further, the rolling load of a stand in the posterior stage of the mandrel mill 1 is detected, and shell tension working on a shell 3 between the stands in a posterior stage is calculated from the rolling load and estimated load torque. Shell tension between the stands in the posterior stage of the mandrel mill 1 is controlled according to a deviation between calculated shell tension and target shell tension by which a stripping abnormality does not occur. Consequently, a bar stripping property can be stabilized.