An integrated circuit device comprising a base structure, a gate stack on the base structure and comprising a plurality of gate electrodes spaced apart from each other, a first upper insulating layer on the gate stack, a plurality of channel structures that penetrate the gate stack, each of the plurality of channel structures comprises a respective alignment key protruding from the gate stack, a second upper insulating layer that overlaps the respective alignment key of each of the plurality of channel structures, a top supporting layer on the second upper insulating layer, a bit line on the top supporting layer, and a plurality of bit line contacts that electrically connect respective ones of the plurality of channel structures to the bit line. A sidewall of the first upper insulating layer includes a first step.