A belt assembly includes a belt formed into an endless loop, a plurality of support rollers with a rotary shaft, a shaft-end retainer, a movable supporting member, and a drive transmission device. The plurality of support rollers includes a first support roller and a second support roller. The shaft-end retainer is disposed at each end of the rotary shaft of the plurality of support rollers. The movable supporting member supports the first support roller and is movably disposed relative to the shaft-end retainer. The drive transmission device transmits a driving force to one of the plurality of support rollers. The drive transmission device and the movable supporting member are disposed on a same side as the shaft-end retainer in an axial direction of the plurality of support rollers. The drive transmission device is disposed between the shaft-end retainer and the movable supporting member in the axial direction.