PURPOSE: To improve durability of an endless belt and eliminate nonuniformity in color and a shearing in printing, by a method wherein a running speed of the endless belt whereon a printing plate is set is synchronized constantly with the line speed of a printer.

CONSTITUTION: A line speed in a printer is detected as a reference speed, while the running speed of an endless belt 14 is detected. Then, the detected running speed of the endless belt 14 is compared with the reference speed, and an acceleration control or a deceleration control of the rotating speed of a plate cylinder 10 is conducted so that the two speeds may be synchronized with each other. Pins 36a of rotary bodies 36 supported axially and rotatably at the opposite ends of the plate cylinder 10 are made to engage removably with numerous through holes 14a made at prescribed intervals at the opposite edges in the longitudinal direction of the endless belt 14. Thereby the deviation of the endless belt 14 in the axial direction in relation to the plate cylinder 10 is prevented.