A sheet stackable device includes a sheet stackable plane. The sheet stackable device includes a first tray and a second tray, each of which is formed in a shape of a flat plate. The first tray includes a first contact part. The second tray is slidable along a slidable direction between a first position, in which the second tray overlaps the first tray, and a second position, in which the second tray is dawn out of the first tray. The second tray includes a second contact part, which is slidable on the first contact part. At least one of the first contact part and the second contact part includes a rack with rack teeth, which are aligned along the slidable direction. An upper plane of the first tray and an upper plane of the second tray in the second position form at least a part of the stackable plane.