To make a nipping roller turn aside by a spring in a state wherein it is disengaged from a plate cylinder, by making a working cylinder operate for the nipping roller, with an idle stroke, to engage the roller with the cylinder.

In the case when an adjusting member constituted as a working cylinder 7 remains at the basic position thereof and a nipping roller 2 is disengaged from a plate cylinder 1, the nipping roller 2 is turned round outward from the plate cylinder 1 on the basis of spring elasticity and a spring 8 is compressed, the head part 5 of a piston rod 6 being raised from the bottom of a bush 4. In other words, the head part 5 of the piston rod 6 thrusts further into the inner chamber of the bush 4. The disengaging operation of the nipping roller 2 from the plate cylinder 1 is transmitted to a control lever 22 operating in coordination with a switch 23, through the intermediary of a lever 3, and then a signal for stopping a machine is delivered.