In the present invention, in cutting the top and bottom surfaces of a hot-rolled sheet by the rotation of milling cutters holding the sheet therebetween in the thickness direction of the sheet in the running process of the hot-rolled sheet, the cutting resistance determined by the cutting conditions including the type of hot-rolled sheet, the temperature in cutting, and the cut depth is controlled by the adjustment of cut depth, and cutting is performed by making the tension produced on the hot-rolled sheet by the cutting resistance lower than the breaking strength of the joint portion to prevent the breakage of hot-rolled sheet.