A nitride semiconductor device includes a substrate; a first nitride semiconductor layer above the substrate; a block layer above the first nitride semiconductor layer; a first opening penetrating through the block layer; an electron transit layer and an electron supply layer provided sequentially above the block layer and along an inner surface of the first opening; a gate electrode provided above the electron supply layer to cover the first opening; a second opening penetrating through the electron supply layer and the electron transit layer; a source electrode provided in the second opening; and a drain electrode. When the first main surface is seen in a plan view, (i) the first opening and the source electrode each are elongated in a predetermined direction, and (ii) at least part of an outline of a first end of the first opening in a longitudinal direction follows an arc or an elliptical arc.