An image forming apparatus has an image reading portion for reading an image of an original and which is disposed in an upper part of the apparatus, an image forming portion for forming an image on a sheet and which is disposed below the image reading portion with a space portion between the image reading portion and the image forming portion, and a sheet delivery portion disposed in the space portion. There is a taper formed on an edge portion of a bottom of the image reading portion defining the space portion so that a sheet treating apparatus for performing treatment such as sorting, alignment, or stapling of sheets on which the images are formed, is mounted in the space portion.