PURPOSE: To improve dimensional accuracy and production efficiency by providing horizontal rolls having web restraining collars and shaft parts and disposing backup rolls and rolls for rolling down flanges which make rolling contact with the shaft parts thereof.

CONSTITUTION: The web restraining collars 7-1 and the shaft parts 7-2 having a taper are provided to the split type horizontal rolls 7. The rolls 7 are rotated by driving shaft 8 and the collars 7-1 grasp and restrain the webs of a rolling material in a rolling process. The backup rolls 9 freely rotatably support work rolls 10 for rolling down the flanges and are displaced in the direction intersecting with the straight line connecting the axial centers of the rolls 7, 10. The rolls 10 are preliminarily driven by being brought into press contact with the shaft parts 7' of the rolls and further one end face thereof contacts with the side walls of the rolls 7. The web is thoroughly restrained by the above-mentioned method and therefore, the dimensional accuracy and production efficiency are improved.