A semiconductor device includes: a substrate; a first nitride semiconductor layer and a second nitride semiconductor layer having a band gap wider than a band gap of the first nitride semiconductor layer; a first active region which includes a source electrode, a drain electrode, and a gate electrode, and has a first carrier layer located in the first nitride semiconductor layer; and a second active region which is on an extension of a long-side direction of the drain electrode and has a second carrier layer located in the first nitride semiconductor layer via an element isolation region, and a potential of the second carrier layer is substantially same as a potential of a source extraction electrode in the second active region or is an intermediate potential between a potential of a gate extraction electrode and the potential of the source extraction electrode opposite a short side of the drain electrode.