To provide an arithmetic unit for retained force for calculating the retained force value of rolling, each time the materials are passed through each roll stand, of a motor for feeding the rolling materials to a mandrel mill.

This arithmetic unit 11 for the retained force is provided with a 1st arithmetic part 11A and 2nd arithmetic part 11B for calculating the retained force value F1 from the calculated value of the torque of a bar retainer driving motor 2A and the acceleration/deceleration torque Ta of a driving machine system, just before biting detecting part 11C for detecting the material just before the rolling material is bitten with the mandrel mill S, a lock-on arithmetic part 11D with which a retained force value F1 is stored and also a retained force F0 just before biting is calculated by removing the torque TL component of the mechanical loss from the stored retained force value F1 when a signal just before the rolling material is bitten into the the mandrel mill S is outputted from the just before biting detecting part 11C, and a rolling retained force arithmetic part 11E for calculating the rolling retained force value F from the retained force value F1 and the retained force value F0 just before biting.