An accessory device detachably attachable to an electronic device comprising a housing which is expandable to define an expanded area of the housing, and a display module which is expandable, together with the housing, to define an expanded area of the display module, and the accessory device may comprise a main body configured to be mounted to the expanded area of the housing, the main body comprising a mounting surface, and a coupling unit which detachably attaches the accessory device to the electronic device, wherein the main body which is attached to the housing which is expanded, may disposes the mounting surface to support the expanded area of the housing together with the expanded area of the display module.