PURPOSE: To expand the shape control range and to roll high hard stocks by using work rolls having a restricted ratio between a reinforcing roll diameter and a work roll diameter and setting concave and convex bending devices on intermediate rolls and the work rolls, respectively.

CONSTITUTION: Small work rolls, for which a ratio between a work roll diameter dw and a reinforcing roll diameter DO is such that dw<0.3DB, are used. Two upper flange parts 34, 36 and two lower flange parts 33, 35 of upper and lower work roll bearing boxes 23, 25 are formed, respectively. Concave bending cylinders 30, 32 and convex bending cylinders 29, 31 are installed in respective spaces between the upper flange parts 34, 36 and lower flange parts 33, 35 of the bearing boxes 23, 25 in project blocks 27, 28 and are positioned at an equal distance from the center of a work roll bearing 43 on the inlet and outlet sides and on a horizontal diagonal line. A device usable for both concave and convex bending is composed of a series of bending cylinders 41 and movable members 42 provided with a hook in shift blocks 37, 38, 39, and 40.