An image forming system comprising: a controller, provided with a list creation means to create a display list by analyzing PDL data 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 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.