A gradation conversion process section converts gradation made up of a plurality of filled regions and filling colors thereof into one-line image data of a filled region into which the plurality of filled regions is unified. A data transmitting process section transmits the one-line image data to a data reception process section. The received one-line image data is interpreted by a PDL interpretation process section, and converted into data which can be interpreted by a rendering process section, and the data is saved on a display list. The rendering process section converts the saved data into gradation made up of a plurality of filled regions and filling colors thereof, and renders the gradation.