PURPOSE: To prevent the generation of a thrust force exerted on a work roll in linkage with rotation of a spindle.

CONSTITUTION: Recessed parts 65a and 65b positioned in the slide surfaces of the nipping pieces 50a and 50b of a slipper metal 51 and closer to the base end part of a fork end part 48 are formed in the fork end part 48 of a coupling 49 on the roll side fixed to the other end part 43 of a work roll 45. Since the nipping pieces 50a and 50b are not moved over the recessed parts 65a and 65b to the base end side of the form end part 48, a stepped surface is not in danger of being generated even when the two surfaces of the fork end part 48 are worn. Thus, the stepped surface is not pressed by the stepped pieces 50a and 50b, and a thrust force is prevented from being generated.