A belt device for an image forming apparatus includes an endless belt member supported at roller members, side frames disposed along sides of the belt member being respectively slidably supported at slide rails disposed at a body of the image forming apparatus, a rear frame constructed across the side frames, rotatably supporting shafts of the roller members at the rear side, a front frame rotatably supporting shafts of the roller members at the front side, and having a projecting surface smaller than an inner periphery of the belt member, and a support frame cantilever-supporting the front frame with respect to the rear frame. The rear frame includes a freely detachable holding member supporting rear side bearings supporting the rear side shafts of support rollers supporting the belt and a sub-bearing which is more to the center side in an axial direction of the support rollers than the rear side bearing.