PURPOSE: To improve the coiling performance of a coil box by operating a roll gap to roll the front end of the material to be rolled to the thickness larger than a target sheet thickness in the final rolling pass of a roughing mill of a hot strip mill.

CONSTITUTION: The coil box coils the material rolled by the roughing mill having a horizontal rolling mill and feeds the un-coiled material to a finishing mill upon completion of the coiling. The front end lA of the material coiled by the coil box is rolled to the sheet thickness hA larger than the target thickness hB of the stationary part lB by operating the roll gap in the final rolling pass of the roughing mill if the thickness of the material subjected to the rolling treatment is small. The coiling performance of the coil box is thereby improved even if the material is thin.