Provided is an image forming apparatus with a preview function and a monochrome/color automatic detection function, capable of displaying a preview of original image data immediately next a detection result has been switched such as from color to monochrome, or monochrome to color, with a simple operation. On receipt of an operation of a next skip button by a user, a preview image display control portion refers to a detection result recording table and displays a preview of image data of a top page in an image data group (unit) that is next the image data being displayed for previewing currently.