To facilitate the conversion from a single color to a full color by respectively providing means for processing image data and bypass means for permitting image data to pass by two sets and providing a means for processing the image data passed through the respective bypass means.

A conventional black and white sensor is substituted with a color sensor 201 and a conventional video pre-processing device is substituted with a video pre-processing device/data stitch circuit 203. Only these substituting operations are required when a monochrome electronic printing machine is converted to a full color electronic printing machine. A chroma image co- processor 213 is added in parallel to a conventional bright image processor 5 and a color image processing sub-system is constituted of these processors. An electronic predetermined gathering memory 7 and a raster output scanner 9 same to a fundamentally usual monochrome printer are used even in a full color printing machine. A light pulse is used to print a document in a full color by a color xerography printing machine 211.