Method for forming intermetal dielectric layer is disclosed including steps of: preparing a substrate with wiring on a lower insulating layer, the wiring having a plurality of separating portions; forming first and second water marks on the lower insulating layer located in the separating portions and on upper surfaces of the wiring; transforming the first and second water marks into first and second air bubbles, respectively; depositing a first insulating layer of lower dielectric constant on the whole surface of the substrate, and at the same time, forming first and second air gaps by growing said first and second air bubbles on and between the wirings, respectively; removing the upper portion of the first insulating layer to make open the second air gap; and depositing a second insulating layer of lower dielectric constant on the first insulating layer to fill the opened second air gap.