A nitride semiconductor device includes: a substrate having a first main surface and a second main surface; a first nitride semiconductor layer of a first conductivity type provided above the first main surface; a second nitride semiconductor layer of a second conductivity type provided above the first nitride semiconductor layer; a first opening which penetrates through the second nitride semiconductor layer to the first nitride semiconductor layer; an electron transport layer provided above the second nitride semiconductor layer and on an inner surface of the first opening; a gate electrode provided above the electron transport layer and covering the first opening; a source electrode connected to the second nitride semiconductor layer; a drain electrode provided on a second main surface-side of the substrate; and a high-resistance layer provided between the second nitride semiconductor layer and the electron transport layer in the first opening, the high-resistance layer including a nitride semiconductor.