An integrated circuit device includes a gate stack structure on a base layer, the gate stack structure having a gate insulating layer with a first dielectric layer on the base layer and having first relative permittivity, and a gate structure on the gate insulating layer, and a gate spacer structure on opposite side walls of the gate stack structure and on the base layer, the gate spacer structure including a buried dielectric layer buried in a recess hole of the gate insulating layer at a lower portion of the gate spacer structure on the base layer, and the buried dielectric layer including a same material as the first dielectric layer.