PURPOSE: To perform quick opening of a gap between main rolls without increasing the capacity of a driving system by making a drafting nut movable in only the axial direction of a screw shaft and making the nut movable in the reverse direction to the drafting direction.

CONSTITUTION: In the case of quickly opening a gap between main rolls, a pressure receiving plate 17 is turned in the reverse direction to the rotating direction by 45° because a drafting screw 9 engages with a worm wheel in only the rotating direction and is freely movable in the screw shaft direction. It is enough to retract the nut 10 by a necessary distance of (x/2) to quickly open the rolls and a retaction speed is controlled by a rotation speed of an arm 21.