A semiconductor memory device includes a substrate including a peripheral circuit; an interconnection array disposed on the peripheral circuit; a cell stack structure disposed on the interconnection array, the cell stack structure including gate electrodes stacked in a vertical direction to form a cell step structure; and a dummy stack structure disposed on the interconnection array, the dummy stack structure including sacrificial layers stacked in the vertical direction to form a dummy step structure parallel to the cell step structure. The interconnection array includes a first lower conductive pattern including a center region overlapping with a slit between the cell step structure and the dummy step structure, a first region extending to overlap with the dummy step structure from the center region, and a second region extending to overlap with the cell step structure from the center region.