A multi-stack semiconductor device includes a substrate, a device isolation layer, first channels, first gate lines covering the first channel, extending in a second horizontal direction, and spaced apart from each other in the first horizontal direction, first source/drain areas arranged on both sides of each of the first channels in the first horizontal direction, a second channel arranged apart from the first gate line in the vertical direction over any one of the first gate lines, a second gate line, second source/drain areas, a third channel arranged apart from the second gate line in the vertical direction over the second gate line, a third gate line, third source/drain areas, and a first lower source/drain contact extending in the vertical direction and connected to each of the first source/drain area, the second source/drain area, and the third source/drain area.