An image forming apparatus includes an image forming unit an original reading unit and a guide mechanism. The image forming unit includes a sheet supply unit; a transportation roller for transporting a medium from the sheet supply unit; an image drum unit for forming a toner image on a photosensitive drum; a transfer belt unit for transferring the toner image on the photosensitive drum to the medium; a fixing device for fixing the toner image to the medium; and a stacker for placing the medium. The original reading unit includes an original stage for placing an original; a scanner unit disposed below the original stage; and a transportation unit for transporting the original to the original stage and a sheet discharge unit. The guide mechanism includes a pair of guide walls integrally formed at upper edges of the image forming unit.