A nitride semiconductor device includes: a conductive substrate; a first semiconductor layer provided on the substrate; a second semiconductor layer provided on the first semiconductor layer; a third semiconductor layer on the second semiconductor layer; a first main electrode connected to the third semiconductor layer; a second main electrode connected to the third semiconductor layer; and a control electrode provided on the third semiconductor layer. The first semiconductor layer is made of AlXGa1−XN (0≦X≦1) of a first conductivity type. The second semiconductor layer is made of a first nitride semiconductor. The third semiconductor layer is made of a second nitride semiconductor which is undoped or of n-type and has a wider bandgap than the first nitride semiconductor.