Nitride semiconductor device includes: a substrate; a first nitride semiconductor layer of a first conductivity above the substrate; a second nitride semiconductor layer of a second conductivity different from the first conductivity, above the first nitride semiconductor layer; a first opening penetrating through the second nitride semiconductor layer; an electron transport layer and an electron supply layer disposed along inner surfaces of the first opening, in stated sequence from the substrate-side; a gate electrode above the electron supply layer, covering the first opening; a source electrode connected to the electron supply layer and the electron transport layer, at a position separated from the gate electrode; and a drain electrode on a surface of the substrate opposite to a surface on which the first nitride semiconductor layer is disposed. At least part of the second nitride semiconductor layer is fixed to a potential different from a potential of the source electrode.