To cope with deformation in width that is difficult to grasp and predict and to reduce error in the width of a hot rolled steel sheet which is actually rolled.

In a hot-rolling mill in which a vertical mill 2 for rolling a rolled stock 1 in the width direction and horizontal mill 3 for rolling the rolled stock 1 in the thickness direction on the downstream side of the vertical mill 2 are arranged as a set, this rolling mill is provided with a target setting means 4 for respectively setting each target value of the width and thickness on the inlet side of the vertical mill 2, each target value of the width and thickness on the outlet side of the horizontal mill 3 and each roll radius of the vertical mill 2 and horizontal mill 3, means 5 for setting the roll opening degree for setting the opening deree between the rolls of the vertical mill 2 so that the width after horizontal rolling is made to coincide with the target value of width, width measuring means 6 for measuring the actual value of width after horizontal rolling, learning means 7 for learning the deviation between the measured actual value of width after horizontal rolling and the target value of width and correcting means 8 for calculating the correction amount of the opening degree between the vertical rolls of the vertical mill 2 from the learned deviation and correcting the opening degree between the vertical rolls of the vertical mill 2 which is set by the means 5 for setting the roll opening degree.