Provided is an image forming apparatus having a preview function and an original paper size automatic detection function, capable of displaying a preview of original image data immediately next a detected original paper size has been switched 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 paper size recording table and displays a preview of image data of a top page in the group (unit) next to the group in which the preview of the image data of the top page is currently displayed, the group being different in a paper size from the other group.