A semiconductor device may include: a first gate structure in a first memory block and including stacked first gate lines, the first gate lines extending from a plane center region to a plane edge region; a second gate structure in a second memory block adjacent to the first memory block and including stacked second gate lines, the second gate lines extending from the plane center region to the plane edge region; an isolation insulating structure located a) between the first gate structure and the second gate structure and b) in the plane edge region, the isolation insulating structure including stacked insulating plates and insulating pillars extending through the insulating plates; and a slit structure located a) between the first gate structure and the second gate structure and b) in the plane center region, the slit structure connected to the isolation insulating structure, and including irregularities on a sidewall thereof.