An integrated circuit device includes: a conductive line structure including a conductive line and an insulating capping pattern; and an insulating spacer including an inner spacer and a first insulating spacer, the inner spacer and the first insulating spacer on a sidewall of the conductive line structure. The first insulating spacer includes: a slit portion; a lower insulating portion spaced apart from the inner spacer such that a separation distance between a portion of the lower insulating portion and the inner spacer decreases with increasing vertical distance from the substrate; and an upper insulating portion contacting the inner spacer. A method of forming the insulating spacer includes: forming a polymer layer on the inner spacer; forming a first insulating spacer layer which contacts each of the inner spacer and the polymer layer; and forming a first insulating spacer by partially removing the first insulating spacer layer.