A transfer unit of an electrophotographic image forming apparatus. The transfer unit includes a transfer belt which is rotated while contacting a photosensitive drum, a driving assembly having a driving roller supported by a plurality of main frames to rotate the transfer belt, and a steering roller portion supported by the plurality of main frames and which pushes the transfer belt to tighten the transfer belt. The apparatus further includes a transfer backup roller assembly having a plurality of transfer backup rollers which are installed inside the transfer belt to support a plurality of auxiliary frames and support the transfer belt, and a plurality of guide rollers supported by the plurality of auxiliary frames and to guide the transfer belt, and a transfer backup roller ascending and descending portion which ascends and descends the transfer backup roller assembly towards the driving assembly.