1. An image forming apparatus, comprising: an image forming section for forming images on papers based on image data of a document having a plurality of document pages and an additional image which is previously set; a post-processing device for carrying out a post-processing for the papers; a first input section for inputting and setting a division of the post-processing of the document pages; a second input section for setting the additional image so as to be formed on a first document page of the plurality of document pages; and a control section for causing the image forming section to form the additional image on the first document page of each division when the division of the post-processing of the document pages is set by the first input section and when the additional image is set by the second input section so as to be formed on the first document page of the plurality of document pages.