Even from a plurality of small documents, their front-side and back-side images to be formed on a recording paper sheet, with alignment and coincidence of the front-side and back-side images of each document between the front and back sides of the recording paper sheet. In an image forming apparatus, an image processing section detects a front-side image of an original document from a scan area read in a first scan by an image reading section, detects a back-side image of the original document from a scan area read in a second scan, and moves or rotates the front-side image and the back-side image of the original document in terms of a whole image of the scan area so that the front-side image and the back-side image of the original document are brought into alignment and coincidence between a front side and a back side of a recording paper sheet.