To prevent a breakage of a platemaking unit and a damage of a surface of a new plate.

The platemaking unit 35 is movable between an operating position capable of platemaking and a non-operating position capable of replacing the plate in a direction of arrows A-B. A cam 79 having a cutout 79a is provided at a lower end of the unit 35, and a detection switch 24 for detecting the cutout 79a is provided in a frame 9B. When the unit 35 is moved from the operating position in the direction of the arrow B and positioned at the non-operating position, the switch 24 detects that the plate can be replaced.