A method for fabricating a semiconductor device includes: forming a silicon nitride film having a refractive index equal to or larger than 2.2 on a nitride semiconductor layer; and introducing at least one of elements that are oxygen, nitrogen, fluorine, phosphorus, sulfur and selenium into the silicon nitride film, the silicon nitride film including the at least one of elements remaining on the nitride semiconductor layer. The at least one of elements is introduced by a process of exposing the silicon nitride film to plasma including the at least one of elements, a process of ion-implanting the at least one of elements into the silicon nitride film, or a process of thermally diffusing the at least one of elements into the silicon nitride film. The silicon nitride film is formed in contact with a surface of the nitride semiconductor layer.