A semiconductor device includes a stacked structure disposed on a substrate. The stacked structure includes a plurality of gate electrodes. The semiconductor device further includes a first structure disposed on the substrate and passing through the stacked structure, and a second structure disposed on the substrate. The second structure is disposed outside of the stacked structure, faces the first structure, and is spaced apart from the first structure. The first structure includes a plurality of separation lines passing through at least a portion of the plurality of gate electrodes and extending outside of the stacked structure, and the second structure is formed of the same material as the first structure.