A nitride semiconductor device includes an active layer including a first nitride semiconductor layer and a second nitride semiconductor layer which are periodically stacked, the second nitride semiconductor layer having a different composition from a composition of the first nitride semiconductor layer. An energy at a lower edge of a conduction band in the first nitride semiconductor layer is lower than an energy at a lower edge of a conduction band in the second nitride semiconductor layer, and an energy at an upper edge of a valence band in the first nitride semiconductor layer is lower than an energy at an upper edge of a valence band in the second nitride semiconductor layer.