PURPOSE: To always and rapidly exchange a roll in the case of being exchanged with a product having prescribed different principal items by combining protruded end parts of roll center shafts which are coaxially provided so that a sleeve and a shaft are mutually slid in the axial direction and moreover integrally rotated.

CONSTITUTION: The alignment between a shaft 42 and a sleeve 44 correspondent to it is obtained by approaching one to the other. First, a ball 56 is begun to come into contact with a truncated conical end part 52 of the shaft 42. The insertion of the ball is helped by the tapered side surface. Next, when the insertion is begun, the sleeve 44 is continued to slowly rotate. Whereupon, the ball 56, which is energized by a spring 58 toward a tapered end part whose outside surface has a spline shape 48, is entered into one between two adjacent spline projected lines 51 to settle. Thus, the positioning between a spline shape 46 of an inside surface and the spline shape 48 of the outside surface is surely executed, and the combination slidable in the axial direction is achieved.