A semiconductor device is provided. The semiconductor device includes a stack structure disposed on a lower structure; an insulating structure disposed on the stack structure; and a vertical structure extending in a direction perpendicular to an upper surface of the lower structure and having side surfaces opposing the stack structure and the insulating structure. The stack structure includes interlayer insulating layers and gate layers, alternately stacked, and the insulating structure includes a lower insulating layer, an intermediate insulating layer on the lower insulating layer, and an upper insulating layer on the intermediate insulating layer.