A nitride semiconductor device includes: a nitride semiconductor structure portion including a first layer made of an n-type group III nitride semiconductor, a second layer made of a group III nitride semiconductor containing a p-type impurity provided on the first layer and an n-type region formed on a part of the second layer, and having a wall surface extending over the first layer, a body region of the second layer other than the n-type region and the n-type region; a gate insulating film formed such that the gate insulating film is opposed to the body region on the wall surface; a gate electrode formed such that the gate electrode is opposed to the body region through the gate insulating film; a source electrode formed such that the source electrode is electrically connected to the n-type region; a drain electrode formed such that the drain electrode is electrically connected to the first layer; and a body electrode formed such that the body electrode is electrically connected to the body region.