A semiconductor device includes a plate layer, gate electrodes and interlayer insulating layers alternately stacked on the plate layer in a first direction perpendicular to an upper surface of the plate layer and forming a first stack structure and a second stack structure on the first stack structure, a channel structure penetrating through the gate electrodes and extending in the first direction, and a contact plug extending in the first direction and electrically connected to one of the gate electrodes, wherein the second stack structure includes a first gate electrode on a lowermost portion, a first interlayer insulating layer on the first gate electrode, and a second interlayer insulating layer on the first interlayer insulating layer, and the first interlayer insulating layer has a first thickness, and the second interlayer insulating layer has a second thickness smaller than the first thickness.