A nitride semiconductor device includes an n-type semiconductor layer, p-type semiconductor layer and an active layer of a quantum well structure that is sandwiched between said p-type and n-type nitride semiconductor layer, wherein the active layer has a first barrier layer, a second barrier layer and a third barrier layer. The first barrier layer is nearest to the p-type nitride semiconductor layer among the first, second and third barrier layers. The second barrier layer is nearest to the n-type nitride semiconductor layer among the first, second and third barrier layers. The third barrier layer is between the first and second barrier layers, and includes an upper barrier layer that contacts with a p-side surface of the well layer and a lower barrier layer that contacts with an n-side surface of said well layer. The upper and lower barrier layers having different composition or impurity concentrations.