A semiconductor device includes a plurality of first gate electrodes sequentially stacked on a substrate, a second gate electrode on the plurality of first gate electrodes, a first channel structure extending through the plurality of first gate electrodes and a portion of the second gate electrode, a buried insulation pattern on a sidewall of the first channel structure, of which an upper surface is at a higher level than a top end of the first channel structure, a second channel structure extending through a remainder of the second gate electrode, the second channel structure connected to the first channel structure, and a buried conductive pattern on a sidewall of the second channel structure.