A semiconductor device is provided with, a group-III nitride semiconductor layered structure that includes a heterojunction, an insulating layer which has a gate opening that reaches the group-III nitride semiconductor layered structure and which is disposed on the group-III nitride semiconductor layered structure, a gate insulating film that covers the bottom and the side of the gate opening, a gate electrode defined on the gate insulating film inside the gate opening, a source electrode and a drain electrode which are disposed to be spaced apart from the gate electrode so as to sandwich the gate electrode, a first conductive layer embedded in the insulating layer between the gate electrode and the drain electrode, and a second conductive layer that is embedded in the insulating layer above the first conductive layer in a region closer to the drain electrode side than the first conductive layer.