A compound semiconductor substrate which inhibits the generation of a crack or a warp and is preferable for a normally-off type high breakdown voltage device, arranged that a multilayer buffer layer 2 in which AlxGa1-xN single crystal layers (0.6≦X≦1.0) 21 containing carbon from 1×1018 atoms/cm3 to 1×1021 atoms/cm3 and AlyGa1-yN single crystal layers (0.1≦y≦0.5) 22 containing carbon from 1×1017 atoms/cm3 to 1×1021 atoms/cm3 are alternately and repeatedly stacked in order, and a nitride active layer 3 provided with an electron transport layer 31 having a carbon concentration of 5×1017 atoms/cm3 or less and an electron supply layer 32 are deposited on a Si single crystal substrate 1 in order. The carbon concentrations of the AlxGa1-xN single crystal layers 21 and that of the AlyGa1-yN single crystal layers 22 respectively decrease from the substrate 1 side towards the above-mentioned active layer 3 side. In this way, the compound semiconductor substrate is produced.