An image forming apparatus includes a first-image-reading-portion composed of a plurality of line sensors used to read a color image, a second-image-reading-portion composed of at least one line sensor used to read a monochrome image, a first-image-processing-portion which processes a first-image-signal processed and outputted by the first-image-reading-portion, a second-image-processing-portion which processes a second image signal processed and outputted by the second-image-reading-portion, a segmentation portion which outputs a segmentation signal indicative of an attribute of an image in accordance with at least one of the first and second image signals, and a signal switching portion which outputs at least one of a third image signal processed and outputted by the first-image-processing-portion and a fourth image signal processed and outputted by the second-image-processing-portion based on the segmentation signal.