Computer-to-cylinder type lithographic printing method and computer-to-cylinder type lithographic printing apparatus

A computer-to-cylinder lithographic printing apparatus comprising:

• a rotative plate cylinder on which a plate material is to be loaded;
• an image forming unit comprising an inkjet recording unit including a recording head having a plurality of ejecting channels so as to form an image directly on the plate material loaded on said plate cylinder by ejecting an oil-based ink from said recording head, based on image data signals, utilizing an electrostatic field to prepare a printing plate;
• an image data processing and control unit which drives said recording head so that every n'th channel of said recording head is actuated in a common phase;
• a plate cylinder's rotational speed-controlling unit which controls the rotational speed of said plate cylinder to give a surface rotational speed V (mm/sec) of the plate material as represented by the following formula:
• V=25.4×(f×n)/N
• wherein N represents a recording resolution (dots/25.4 mm) along a rotative direction of the plate cylinder on said plate material, and f represents a driving frequency f (Hz) of each ejecting channel of said recording head;
• and
• a lithographic printing unit which performs lithographic printing with the thus prepared printing plate.