Disclosed are an npn-type bipolar transistor as a nitride semiconductor device having good characteristics, and a method of manufacturing the same. A so-called pn epitaxial substrate has a structure wherein an n-type collector layer and a p-type base layer of a three-layer structure are provided over a substrate. The three-layer structure includes first (lower layer side), second, and third (upper layer side) p-type base layers which differ in thickness and p-type impurity concentration. In a partial region inside the second p-type base layer located as an intermediate layer in the p-type base layer of the three-layer structure, an n-type emitter region is formed by ion implantation.