The method of the invention for producing a Group III nitride compound semiconductor, employing an etchable substrate which is produced from a material other than the Group III nitride compound semiconductor, includes stacking one or more layers of the Group III nitride compound semiconductor on one face of the substrate and etching the other face of the substrate while stacking one or more semiconductor layers or after completion of stacking one or more semiconductor layers, to thereby reduce the thickness of most of the substrate. The apparatus of present invention for producing a semiconductor through vapor phase growth, contains a substrate for vapor-phase-growing the semiconductor; a source-supplying system for supplying a source for vapor phase growth of the semiconductor; and an etchant-supplying system, wherein the source-supplying system and the etchant-supplying system are isolated through placement of the substrate.