A semiconductor device includes: a nitride semiconductor layer; a first silicon nitride film that is formed on the nitride semiconductor layer, has a first opening whose inner wall is a forward tapered shape; a second silicon nitride film that is formed on the first silicon nitride film, and has a second opening whose inner wall is an inverse tapered shape; and a gate electrode formed so as to cover the whole surface of the nitride semiconductor layer exposed on the inside of the first opening; wherein a side wall of the gate electrode separates from the first silicon nitride film and the second silicon nitride film via a cavity.