An image forming system, including: an image forming apparatus and a post processing device, wherein the image forming apparatus includes; a printer section, a control means, a selecting device to select one of a single sided print mode and a double sided print mode, and the post processing device includes; a sheet supplying tray, a conveyance route, and a post processing means, wherein the control means controls to count the page number to be assigned onto plural sheets of recording media, using a different counting value, based on the selected print mode.