A method of fabricating a semiconductor device capable of increasing the selectivity of a low dielectric constant insulator film to an etching mask layer such as an etching stopper film without increasing the thickness of the etching mask layer is obtained. This method of fabricating a semiconductor device comprises steps of forming a first insulator film including a polymer film containing C and H, forming a first etching mask layer containing Si on a prescribed region of the first insulator film and plasma-etching the first insulator film with etching gas containing nitrogen and monochromated ion energy having a narrow energy width through a mask of the first etching mask layer.