An integrated circuit device includes: a pair of width-setting patterns over a substrate, the pair of width-setting patterns defining a width of a gate structure space in a first direction and extending in a second direction intersecting with the first direction. A gate electrode layer is provided that extends in the gate structure space along the second direction. A gate insulating layer is provided in the gate structure space and between the substrate and the gate electrode layer. An insulating spacer is provides on the pair of width-setting patterns, the insulating spacer covering both sidewalls of the gate electrode layer, wherein the pair of width-setting patterns have a carbon content that is greater than a carbon content of the insulating spacer.