To protect a free end part of a rubber blanket on a retension plate from unnecessary abrasion by a filling piece of an annexed device by a method wherein a device fixing a rubber blanket unit makes a pollutant and printing ink hardly reach an inside of a narrow slit.

In order to tighten edge bent end parts 3, 4 of a rubber blanket 2, a spindle 22 is rotated counterclockwise until a pressure piece 23 is adjusted almost perpendicularly to leg parts 12, 13 of a retention plate 9. A pressure projection 24 of the pressure piece 23 is fixed by pressure to the leg parts 12, 13 of the retention plate 9 by spring force supported with the spindle 22. At the same time, the slider 23, a filling piece 33, and the leg 13 fixed at a reverse hook 36 of the following edge bent end part 4 are drawn into a slit 8, and the filling piece 33 fills a gap left between the mutually opposed end parts of the rubber blanket 11.