PURPOSE: To effectively prevent buckling of width direction of a slab by inclining whole pressing face of a die toward slab thickness direction at the prescribed angle and specifying setting position of pressing roller for preventing the buckling.

CONSTITUTION: The dies 6 connected with a press 5 are arranged as facing under some interval related to running line L of the slab 1. The pressing faces for slab of the dies 6 are composed of tapered part 61 toward slab advancing direction and paralleled flat part 62 to the advancing direction. And, whole pressing faces of the dies 6 are inclined at the prescribed angle toward thickness direction of the slab 1. This inclined angle is desirable to be upward opening or downward opening at about 0.5W3° And, the pressing roller 7 is set within X=500 mm distance from a point 63 intersection of the tapered part 61 and the flat part 62 toward inlet side of the slab advancing direction at the middle position of dies and lower side of the line L. By this method, at the time of rolling under large reduction of width of the slab, the width sizing can be executed without entirely developing the buckling and the pressing flaw.