A semiconductor device includes a substrate having a cell array region and a pad region, a stack structure including gate electrodes and mold insulating layers alternately stacked on the substrate and having a staircase shape in the pad region, first separation regions penetrating the stack structure in the pad region, extending in a first direction, and including first and second dummy insulating layers, the first dummy insulating layers covering side walls of the first separation regions and including horizontal portions covering portions of the gate electrodes, and the second dummy insulating layers disposed between the first dummy insulating layers, extending portions extending towards the mold insulating layers from the first dummy insulating layers in a second direction perpendicular to the first direction, second separation regions dividing the stack structure and extending in the first direction, and cell contact plugs penetrating the horizontal portions and connected to the gate electrodes.