To easily execute the change of plural calibers, consequently, to improve productivity and operability and also furthermore to make the whole constitution simpler.

A roll housing 7 which is shiftable in the axial direction of work rolls 8 is separately provided from bevel gear boxes 6, by shifting this housing 7, the change of the calibers is executed. To absorb the variation of the length in the axial direction in a driving force transmitting route with the shift of the housing 7, axial extending/contracting means such as a spline shaft 25 is provided in this route. At the time of changing the calibers, detaching and attaching the work rolls 8 is unnecessary. In this way, the work is simplified, necessary time is also shortened and the productivity is improved.