A semiconductor device includes a substrate having a first region and a second region, gate electrodes stacked in the first region and forming a pad region having a stepped shape extending by different lengths in the second region, interlayer insulating layers alternately stacked with the gate electrodes, channel structures passing through the gate electrodes in the first region and including a channel layer, separation regions passing through the gate electrodes in the first and second regions, an etch-stop layer disposed on uppermost gate electrodes, among the gate electrodes forming the pad region in the second region, not to overlap the first region and the separation regions, a cell region insulating layer covering the gate electrodes and the etch-stop layer, and contact plugs passing through the cell region insulating layer and the etch-stop layer in the second region and connected to the gate electrodes in the pad region.