Abstract: A nitride semiconductor substrate suitable for a high withstand voltage power device is provided in which current collapse is controlled, while reducing leakage current. In a nitride semiconductor substrate, wherein a buffer layer, an active layer, and an electron supply layer, each comprising a group 13 nitride, are stacked one by one on a silicon single crystal substrate, the buffer layer has a structure where a multilayer stack in which a pair of nitride layers having different concentrations of Al or Ga are repeatedly deposited a plurality of times on an initial layer of AlxGa1?xN (0?x?1) is stacked, and includes a doping layer whose carbon concentration is 1×1018 to 1×1021 cm?3 and whose Si concentration is 1×1017 to 1×1020 cm?3, a thickness of the doping layer is 15% or more of the total thickness of the buffer layer.