A nitride semiconductor device according to the present invention includes: a nitride semiconductor laminated structure comprising a first layer made of a Group III nitride semiconductor, a second layer laminated on the first layer and made of an Al-containing Group III nitride semiconductor with a composition that differs from that of the first layer, the nitride semiconductor laminated structure comprising a stripe-like trench exposing a lamination boundary between the first layer and the second layer; a gate electrode formed to oppose the lamination boundary; and a source electrode and a drain electrode, having the gate electrode interposed therebetween, each connected electrically to the second layer.