To make the precision of the width of the crude bar satisfactory, which is subjected to width rolling down in the rough rolling line.

When the absolute value of a difference obtained by subtracting a crude bar target width Wba from the crude bar detected width Wb of the crude bar S2 after the crude bar S2 has been subjected to horizontal rough rolling by a roughing mill 8 existing in the downstream of the feed direction D1 of a material, is smaller than a crude bar width error set value WO, by adjusting the roll gaps of edgers 3, 4 and 5, the width rolling down quantity of the crude bar S2 is corrected for a slab S1 to be subjected to the width rolling down for a next time. When the absolute value of a difference obtained by subtracting the crude bar target width Wba from the crude bar detected width Wb of this crude bar S2, is larger than a crude bar width error set value WO, the width set quantity of a sizing press 2 is adjusted. Thus the width rolling down quantity of the slab S1 is corrected for the slab S1 to be subjected to the width rolling down for a next time, and by adjusting the roll gaps of the edgers 3, 4 and 5 on the upstream side in the feed direction of the material from the width detecting position of the crude bar, the width rolling down quantity of the crude bar S2 is corrected for the slab S1 to be subjected to the width rolling down for a next time.