A self-supported III-V nitride semiconductor substrate having a substantially uniform carrier concentration distribution in a surface layer existing from a top surface to a depth of at least 10 μm is produced by growing a III-V nitride semiconductor crystal while forming a plurality of projections on a crystal growth interface at the initial or intermediate stage of crystal growth; conducting the crystal growth until recesses between the projections are buried, so that the crystal growth interface becomes flat; and continuing the crystal growth to a thickness of 10 μm or more while keeping the crystal growth interface flat.