To obtain an image with good visibility by identifying an original document as being a color original document or a monochrome original document for every original document and using processing suitable for each for an image read by the image reading device.To achieve the object, the invention includes: a color processing portion 77 that performs color image forming processing for a multi-color color original document made of more than one color; a color original document monochrome processing portion 74 that performs monochrome image forming processing for a monochromatic color original document made of a single color; a monochrome original document processing portion 73 that performs monochrome image forming processing different from the monochrome image forming processing by the color original document monochrome processing portion 74 for a monochrome original document; and a CPU 75 that enables appropriate selective use of the color processing portion 77, the color original document monochrome processing portion 74, and the monochrome original document processing portion 73 for each original document.