PURPOSE:To decrease fishtails without promoting the width reduction in leading and tail end parts by press-forming the part apart by more than a specific distance from the end face to narrow width then applying broadside screw down and thickness screw down in the end parts in the longitudinal direction of a slab. CONSTITUTION:A pair of press heads 1 for broadside screw down moved forward and backward in the width direction by means of oil hydraulic cylinders 2 are provided to the front stage of rolling mills. The part apart by a distance (l) from the end face E of a slab S is press-formed to narrow width by the heads. However, the end part S1 is not subjected to edging. Namely, the distance (l) is at least the distance at which at least said end part S1 can retain the original shape. Then, the end part E forms the shape wherein the central part in the width direction overhangs. Next, when such slab S is rolled with rolling mills of the rear stages, the end part S1 forms a fishtail, but it is decreased considerably as it is offset by the overhanging effect of the central part in the width direction. Since the end part S1 is not press-formed, the width reduction hardly occurs.