To decrease a development load of a system capable of using an option controller.

A main controller 14 of this image forming apparatus 1 executes a process that depends on a print engine 11. A relay device 15 receives raster image data from an option controller 3 to be stored in a memory 42, receives a control command based on a result of the process that depends on the print engine 11 from the main controller 14, reads the raster image data from the memory 42 in accordance with the control command, and supplies the read raster image data to the print engine 11.