An image forming apparatus assembly includes a main body, and a belt unit mounted to the main body and including an endless belt, a plurality of rollers including a first roller configured to apply a driving force to the belt and to stretch the belt, and a second roller around which the belt is wound with a wrapping angle not less than 90° and configured to stretch the belt, a frame configured to integrally support the rollers, a first engaging portion disposed in the frame and configured to position the first roller to the main body, and a second engaging portion disposed in the frame and configured to position the second roller to the main body. In addition, a guide member is configured to guide the belt unit and disposed in the main body, and including a first engaged portion engaged with the first engaging portion and a second engaged portion engaged with the second engaging portion.