An image forming system comprising: a controller, provided with a list creation means to create a display list by analyzing PDL data, a decompression means to decompress compressed image data, a drawing means to execute drawing based on the image data, and a print data creation means to create the print data based on the image data, for sending the print data to a printer; and a rasterization accelerator, provided with a second drawing means to execute drawing processing based on the display list, and a compression means to compress the image data drawn by the second drawing unit, for sending the image data to the controller, wherein the rasterization accelerator comprises a determination means for determining, based on the information of the display list, whether to execute or not the compression of the image data, and a compression method in case of executing the compression.