A nitride semiconductor substrate is provided which is suitable for a high withstand voltage power device and prevents a warp and a crack from generating in a Si substrate when forming a thick nitride semiconductor layer on the substrate. A nitride semiconductor substrate 1 is prepared in such a manner that a buffer layer 3 and a semiconductor active layer 4 each comprising a group 13 nitride are stacked one by one on one principal plane of a Si single crystal substrate, the one principal plane has an offset angle of 0.1° to 1° or −1° to −0.1° with respect to a (111) plane, an average dopant concentration in a bulk is 1×1018 to 1×1021 cm−3, the Si single crystal substrate 2 has a SiO2 film on the back, and the total thickness of the buffer layer 3 and the semiconductor active layer 4 is 4 to 10 μm.