A bearing 4 for a set on/off roller 2 compensates for the channel shock force 19 of cylinder 1 by setting the direction of action of a resultant force 15 (formed by a combination of the force 16 of an actuation device 6, the force 17 of a roller 9 and the gravity force 18 of the roller 2) to act in the same direction as that of the shock force.