A semiconductor device includes an upper-level layer having a cell array region, a cell contact region and a dummy region on a substrate. The upper-level layer includes a semiconductor layer, a cell array structure including first and second stack structures sequentially stacked on the semiconductor layer of the cell array region, the first and second stack structures comprising stacked electrodes, a first staircase structure on the semiconductor layer of the cell contact region, the electrodes extending from the cell array structure into the first staircase structure such that the cell array structure is connected to the first staircase structure, a vertical channel structure penetrating the cell array structure, a dummy structure in the dummy region, the dummy structure at the same level as the second stack structure, the dummy structure including stacked first layers, and cell contact plugs in the cell contact region and connected to the first staircase structure.