PURPOSE: To roll an extremely wide and extremely thick steel plate, by installing an extremely wide steel plate finishing mill accompanied by a cooling apray and a down-coiler in the line connecting the cutting machine with the roughing mill, and by equalizing the rollable widths of the roughing mill, the two-high rolling mill and the extremely wide steel plate finishing mill.

CONSTITUTION: In the rolling line connecting the roughing mill 3 with the cutting machine 4, an extremely wide steel plate finishing mill 8 is installed, and its accompaniments, a cooling spray 9 and a down-coiler 10 are arranged so as to be insertable into and removable from the rolling line. Moreover, the rollable widths of the roughing mill 3 and the two-high rolling mill 2 are made equal to the rollable width of the finishing mill 8 for the extremely wide steel plate. The slab 2, extracted from the heating furnace 1, is rolled by the two-high rolling mill 2, and next by the roughing mill 3; after that, the slab 2 is finished by the finishing mill 8 into a plate of desired breadth and thickness, which is then cooled by the cooling spray 9. This plate is coiled by the down-coiler 10, and a specifically sized, extremely wide and extremely thick steel plate is obtained.