In an image forming apparatus, an image scanning device scans a document image of a document. An image type identifying unit identifies a type of the scanned document image. An operation detecting unit detects an image quality adjustment operation by a user. A preview processing unit selects one of whole preview displaying and partial preview displaying on the basis of the type of the document image and a type of the image quality adjustment operation, and performs the selected whole preview displaying or partial preview displaying. The whole preview displaying indicates a whole of the document image, and the partial preview displaying indicates a part of the document image at a higher resolution than a resolution of the whole preview displaying. An image processing unit performs image quality adjustment of the document image according to the detected image quality adjustment operation.