An image forming apparatus includes an image forming unit to form an image on a sheet of recording media, an image reading unit to read image data of an original document, disposed above the image forming unit, a sheet discharge space defined by the image reading unit and the image forming unit and disposed therebetween, a discharge port formed in a first inner side wall of a main body of the image forming apparatus, through which the sheet is discharged into the sheet discharge space, a positioning member to position and detachably mount a positioning member to position and detachably mount an auxiliary unit adjacent to the discharge port in the sheet discharge space.