To sufficiently make the most of processing capacity of an engine when a file size after compression exceeds a stipulated value in an image forming system provided with an FEP for performing RIP processing and compression processing and a BEP for controlling the engine.

A size information extracting part 682 extracts a compression data size by referring to header information. A compression rate calculating part 683 calculates an actual compression rate. A synchronous propriety deciding part 686 compares the calculated compression rate and a stipulated compression rate fixed in relation to the engine 30. When the compression rate is larger than the stipulated compression rate, the synchronous propriety deciding part 686 requests an FEP part 500 to perform a recompression processing. A compression parameter set to a compression processing part 530 is changed by a file size control part 570 to make the compression rate smaller. The compression processing part 530 reexecutes the compression processing by using an adjusted compression parameter and retransfers the processed compression image data to a BEP part 600.