The present invention provides a semiconductor device in which reduction of on-resistance or suppression of current collapse is achieved and reduction of voltage endurance is prevented. The semiconductor device includes: a first semiconductor layer formed on a substrate and formed of a nitride-based semiconductor; a second semiconductor layer formed on a surface of the first semiconductor layer and formed of a nitride-based semiconductor having a wider band-gap than the first semiconductor layer; first and second electrodes formed on a surface of the second semiconductor layer; an inter-electrode insulator film that is formed between the first and second electrodes on the surface of the second semiconductor layer; and a dielectric constant adjustment layer formed on the inter-electrode insulator film and formed of an electric insulator. The first electrode has a field plate portion formed so as to ride on the inter-electrode insulator film, and the dielectric constant adjustment layer has a first dielectric constant adjustment layer that contacts a lateral end portion of the field plate portion and a second dielectric constant adjustment layer formed on the first dielectric constant adjustment layer.