An integrated circuit device includes a plurality of word lines, a string selection line structure stacked on the plurality of word lines, and a plurality of channel structures extending in a vertical direction through the plurality of word lines and the string selection line structure. The string selection line structure includes a string selection bent line including a lower horizontal extension portion extending in a horizontal direction at a first level higher than the plurality of word lines, an upper horizontal extension portion extending in the horizontal direction at a second level higher than the first level, and a vertical extension portion connected between the lower horizontal extension portion and the upper horizontal extension portion.