A semiconductor substrate includes a first semiconductor layer that is formed on a semiconductor base substrate, a second semiconductor layer that is formed on the first semiconductor layer and that has an etching selection ratio smaller than that of the first semiconductor layer, a cavity portion that is formed below the second semiconductor layer by removing a portion of the first semiconductor layer, a thermal oxidation film that is formed on the surface of the second semiconductor layer in the cavity portion, and a buried insulating film that is buried in the cavity portion.