To form a good image surely by preventing the irregular reflection of light beams in a digital printing machine for forming the image by directly exposing a printing plate wound onto a plate cylinder.

The printing plate is fixed to the periphery of the plate cylinder by a catching means arranged in a cylinder groove. The periphery of the cylinder groove is made an image forming prohibition area, and its coordinate is stored in an internal memory. A dummy data setting part inserts dummy data into image data in comparison with information stored in the internal memory to prevent the periphery of the cylinder groove from being exposed to light beams.