A semiconductor device includes a substrate having a cell region and a connection region adjacent to the cell region. A lower stack structure and an upper stack structure are disposed on the substrate. A channel structure is provided to pass through the upper stack structure and the lower stack structure. A distance between a lower extension line portion included in an uppermost one of a plurality of lower interconnection layers and an upper extension line portion included in a lowermost one of a plurality of upper interconnection layers is less than a distance between a lower gate electrode portion included in the uppermost one of the plurality of lower interconnection layers and an upper gate electrode portion included in the lowermost one of the plurality of upper interconnection layers.