A sheet treating apparatus is mountable in a space portion of an image forming apparatus including an image-forming portion for forming an image on a sheet and an image-reading portion for reading an image of an original. The image-reading portion is disposed above the image-forming portion. The sheet is delivered in the space portion formed between the image-forming portion and the image-reading portion. The sheet treating apparatus includes a connection path unit to be connected with a sheet delivery path for delivering the sheet from the image-forming portion, which connection path unit is mounted in the space portion from the outside of a side surface of the main body of the image forming apparatus, and a sheet-treating unit for performing a treatment of the sheet transported from the connection path unit, which sheet-treating unit is mounted in the space portion from the front side of the main body of the image forming apparatus.