To improve the straightening effect and the reducing effect of the residual stress by giving the slight rolling reduction function to a horizontal roller of a roller straightening machine, and straightening a work to be straightened through slight rolling reduction when the work is slightly reduced.

A work 20 to be straightened is straightened by a roller straightening machine 1. Rolls 3, 4 on the inlet side and rolls 10, 11 on the outlet side are provided with a roll pitch changing device so that the slight rolling reduction can be performed by moving an upper roll and a lower roll to approximately same position. Intermediate rolls 5-9 are located in a staggered manner. The rolls 3, 4, 10, 11 can be used in a staggered arrangement in a normal condition. When a work 20 to be straightened with large bend is straightened, the rolls 3, 4, 10, 11 are moved by the roll pitch changing device to set the upper and lower rolls opposite to each other to realize the arrangement of a slight rolling reduction mill. The large bend of the work 20 is straightened by the rolls 3, 4 on the inlet side, and further straightened by the repeated bend by the intermediate rolls 5-9 arranged in a staggered manner, and the whole work is straightened by the rolls 10, 11 on the outlet side.