In an image forming apparatus provided with an image reader to read an image on an image surface of a document and to output image data corresponding to the image; an image processor to process the image data; an image forming device to successively form images on sheets on the basis of the processed image data; there are further provided a post processing device having a stapling stand on which the sheets are placed and a stapler to staple at a predetermined position on the stapling stand, wherein the predetermined position locates at only a single side of the sheet placed on the stapling stand; and a stapling position selector to select a stapling position on the image formed on the sheets, wherein the image processor processes the image data so that when the sheets are placed on the stapling stand, the stapling position on the image formed on the sheets selected by the stapling position selector coincides with the predetermined position on the stapling stand.