A semiconductor device includes first and second gate electrodes stacked and spaced apart from each other in a first direction on a first region of a substrate, and extending in staircase form in a second direction on a second region of the substrate, the second gate electrodes disposed on the first gate electrodes; a first support structure penetrating the first gate electrodes on the second region, extending in the first direction, and having an upper end disposed at a level lower than a level of a lowermost second gate electrode among the second gate electrodes; a second support structure penetrating at least one of the first and second gate electrodes on the second region, extending in the first direction, and having an upper end disposed at a level higher than a level of un uppermost second gate electrode among the second gate electrodes.