An image forming apparatus includes a display having a function of editing input image data and a function of displaying page images immediately after reading in order. When a first type of instruction to change shape of an image formed on a recording medium is given, the edition is not reflected on a document display mode screen image, and when a second type of instruction to edit page by page the recording medium having images formed thereon is given, the edition is reflected on the document display mode screen image. By switching between a finish preview screen image and the document display mode screen image, the image after edition and the image immediately after reading can be compared easily.