A positioning structure includes a positioning aiding member attached to a horizontal beam of a sheet feeder and having a positioning hole formed at an opposed portion thereof opposed to the horizontal beam. The positioning aiding member has a guide part that, when an image forming apparatus is installed onto a top of the sheet feeder, guides a horizontal beam of the image forming apparatus such that a positioning hole formed in a lower surface of the horizontal beam of the image forming apparatus approaches a first positioning pin coupled with the positioning aiding member as viewed in an extending direction of the horizontal beam of the sheet feeder.