A belt unit includes a belt that is endless, a plurality of stretching rollers, a pair of side frames and one or more shafts. The side frames are each provided with bearing holes that rotatably support opposite end parts of the stretching rollers, and positioning holes into which opposite end parts of the shafts are inserted. The shafts each have formed therein a first engagement groove that engages with a first holding jig. The stretching rollers each have formed therein a second engagement groove that engages with a second holding jig. When the first engagement groove is in engagement with the first holding jig, the shafts are arranged opposite the positioning holes corresponding thereto, and have a phase thereof restricted in a circumferential direction thereof. When the second engagement groove is in engagement with the second holding jig, the stretching rollers are arranged opposite the bearing holes corresponding thereto.