An image forming apparatus includes a controller having an integrated circuit for image processing. The controller is connected to an engine via a peripheral component interconnect (PCI). The engine includes a plotter and a scanner. The controller includes a central processing unit (CPU) to which a chip-set is connected via an accelerated graphics port (AGP). The controller also includes an application specific integrated circuit (ASIC) that controls whether to output scanner image data, which is data acquired by the scanner, to the PCI as plotter data for the plotter, or to output the scanner image data to the AGP, or to output image data input through the AGP to the PCI as plotter data for the plotter. The ASIC includes a combiner that combines a plurality of image data.