A compound semiconductor substrate 10 according to the present invention is comprised of a Group III nitride and has a surface layer 12 containing a chloride of not less than 200×1010 atoms/cm2 and not more than 12000×1010 atoms/cm2 in terms of Cl and an oxide of not less than 3.0 at % and not more than 15.0 at % in terms of O, at a surface. The inventors conducted elaborate research and newly discovered that when the surface layer 12 at the surface of the compound semiconductor substrate 10 contained the chloride of not less than 200×1010 atoms/cm2 and not more than 12000×1010 atoms/cm2 in terms of Cl and the oxide of not less than 3.0 at % and not more than 15.0 at % in terms of O, Si was reduced at an interface between the compound semiconductor substrate 10 and an epitaxial layer 14 formed thereon and, as a result, the electric resistance at the interface was reduced.