To simplify a hydraulic system, to reduce the cost of equipment and to facilitate maintenance and inspection by setting respectively simultaneously positions of a mill stand and a spindle holding device with a shifting cylinder.

At the time of changing a rolling size, when only the mill stand 2 is moved to the middle side of a line, by advancing the rod 5a of the shifting cylinder 5, the mill stand 2 is moved to the middle side of line in the state the spindle holding device 3 is stopped. Besides, when the rod 5a is retreated, the spindle holding device 3 is moved to the drive side by engaging an engaging block 6 with a receiving block. In such a case, the mill stand 2 and the spindle supporting device 3 are unitedly shifted in the direction away from the middle of the line.