A semiconductor device of the disclosure includes a peripheral circuit structure including a peripheral transistor, a semiconductor layer on the peripheral circuit structure, a source structure on the semiconductor layer, a gate stack structure on the source structure, the gate stack structure including a word line, a gate upper line and a staircase structure, a memory channel structure and a dummy channel structure extending through the gate stack structure, a cut structure extending through the gate upper line, and a bit line overlapping with the memory channel structure. The cut structure includes a narrow section, and a wide section nearer to the staircase structure than the narrow section. A width of the narrow section is less than a width of the wide section.