In a method for manufacturing a semiconductor device, a first dielectric layer is formed over a substrate, first recesses are formed in the first dielectric layer. Metal wirings extending is a first direction are formed in the first recesses. A mask layer is formed over the metal wirings and the first dielectric layer, which includes a first opening extending in the first direction and is located above a space between adjacent two metal wirings. A first groove corresponding to the first opening is formed between the adjacent two metal wirings by etching the first dielectric layer using the mask layer as an etching mask. A second dielectric layer is formed so that a first air gap is formed in the first groove. A width of the first opening in a perpendicular direction to the first direction is smaller than a space between the adjacent two metal wirings.