The invention provides a nitride semiconductor device and a manufacturing method thereof. In the invention, n-type and p-type nitride semiconductor layers are formed on a substrate, and an active layer is formed therebetween. The n-type nitride semiconductor layers include first and second n-type GaN layers disposed in the order of distance from the active layer. In addition, in the nitride semiconductor device of the invention, an AlxGal-xN layer, where 0