A semiconductor device including: a substrate including a top surface configured to extend in a first direction and a second direction that are perpendicular to each other; gate stack structures disposed on the substrate, spaced apart from one another in the first direction and configured to extend in the second direction; a first region in which levels of top surfaces of the gate stack structures are constant; a second region in which levels of top surfaces of the gate stack structures are stepped, the second region configured to surround at least a portion of the first region; and conductive lines disposed in the second region between the gate stack structures and configured to extend in the second direction in an uneven form.