A semiconductor device including a substrate; a horizontal conductive layer disposed on the substrate; a support layer disposed on the horizontal conductive layer; a stack structure including a plurality of gate electrodes, stacked to be spaced apart from each other in a direction perpendicular to an upper surface of the support layer, and a plurality of interlayer insulating layers stacked alternately with the plurality of gate electrodes; a channel structure penetrating through the stack structure; a separation structure penetrating through the horizontal conductive layer, the support layer, and the stack structure and extending in a first direction; and a conductive pattern disposed on a level between the horizontal conductive layer and a lowermost interlayer insulating layer, among the plurality of interlayer insulating layers, and protruding outwardly of the separation structure from a side surface of the separation structure.