An integrated circuit device includes: a semiconductor substrate having a cell region and a dummy region outside the cell region, a plurality of gate electrodes and a plurality of insulating layers, in the cell region, extending in first and second directions parallel to a main surface of the semiconductor substrate and alternately stacked in a third direction perpendicular to the main surface of the semiconductor substrate, the first and second directions crossing each other, and a plurality of dummy mold layers and a plurality of dummy insulating layers alternately stacked in the dummy region in the third direction, wherein a carbon concentration of an upper dummy mold layer of the plurality of dummy mold layers is less than a carbon concentration of a lower dummy mold layer of the plurality of dummy mold layers, the lower dummy mold layer being between the upper dummy mold layer and the main surface of the semiconductor substrate.