An add-on storage device for flat objects, particularly CD and/or DVD cases, is made up of box-shaped housing elements which are open at the front and lie against one another with facing side walls. Clip-shaped connection elements are pushed onto adjoining side walls which form a common edge, in a predetermined fastening region from the open housing side and from the opposite bottom side, to engage over the side walls at the inner surface of the housing elements. In the fastening region, the open side at the inner surfaces of the adjoining side walls have a depression and/or elevation, and the connection element has an associated elevation and/or depression. The elevation and/or depression of the inner surface of the side wall forms a positive lock-in connection with the associated depression and/or elevation at the connection element.