An integrated circuit device includes a plurality of conductive lines extending in a horizontal direction parallel to a main surface of a substrate and overlapping one another in a vertical direction vertical to the main surface, on the substrate, a plurality of insulation layers each between two adjacent conductive lines of the plurality of conductive lines to extend in the horizontal direction, a channel layer extending in the vertical direction in a channel hole passing through the plurality of conductive lines and the plurality of insulation layers, and a plurality of outer blocking dielectric layers between the plurality of conductive lines and the channel layer, in at least some of the plurality of conductive lines, wherein a width of each of the plurality of outer blocking dielectric layers in the horizontal direction increases toward the main surface.