To expand the kind of an engine connectable to a BEP in an image forming system provided with an FEP for performing a RIP processing and a BEP for controlling the engine.

An output destination information extracting part 627 extracts output destination information from header information of a printing file from the FEP part 500. A frequency selector 629 collates a video clock frequency requested by the connected engine acquired by a video clock information acquiring part 628 with output destination information extracted by the output destination information extracting part 627 to determine a clock frequency VCLK of image data to be transferred to the engine designated by the FEP part 500. A printing controlling part 620 sets the determined video clock frequency VCLK to each part in the BEP part 600. Each part in the BEP part 600 performs a prescribed processing at a processing speed suitable for an output destination engine. An interface part 650 transmits image data to an engine side at a rate of the video clock frequency VCLK.