A semiconductor device is disclosed. The semiconductor device may include gate stacks that are on a substrate, are spaced apart from each other in a first direction, and include electrodes and cell insulating layers alternately stacked, a separation structure between the gate stacks and extending in a second direction crossing the first direction, vertical structures penetrating the gate stacks and having conductive pads on upper portions thereof, a supporting structure on the gate stacks, bit lines on the supporting structure, and contact plugs penetrating the supporting structure and electrically connecting the bit lines to the vertical structures. A bottom surface of a portion of the supporting structure on the separation structure may be lower than top surfaces of the conductive pads.