A semiconductor device includes an insulating structure; a plurality of horizontal layers vertically stacked and spaced apart from each other in the insulating structure; a conductive material pattern contacting the insulating structure; and a vertical structure penetrating through the plurality of horizontal layers and extending into the conductive material pattern in the insulating structure. Each of the plurality of horizontal layers comprises a conductive material, the vertical structure comprises a vertical portion and a protruding portion, the vertical portion of the vertical structure penetrates through the plurality of horizontal layers, the protruding portion of the vertical structure extends from the vertical portion into the conductive material pattern, a width of the vertical portion is greater than a width of the protruding portion, and a side surface of the protruding portion is in contact with the conductive material pattern.