A semiconductor device includes: an electron transit layer formed on a substrate and of a group III nitride-based compound semiconductor; an electron supply layer formed on the electron transit layer and of a group III nitride-based compound semiconductor having a higher band gap energy than the transit layer; a field plate layer formed on the supply layer, formed of a non-p-type group III nitride-based compound semiconductor, and having a lower band gap energy than the supply layer; a first electrode forming an ohmic contact with a two-dimensional electron gas layer in the transit layer at an interface thereof with the supply layer; and a second electrode forming a Schottky contact with the electron gas layer. The second electrode forms an ohmic contact, at a side wall of the field plate layer, with two-dimensional hole gas in the field plate layer at an interface thereof with the supply layer.