A method for fabricating a Group III nitride semiconductor substrate according to the present invention includes the steps of: (a) preparing a substrate; (b) forming, on the substrate, a first semiconductor layer composed of a Group III nitride semiconductor; (c) forming, on the first semiconductor layer, a heat diffusion suppressing layer lower in thermal conductivity than the first semiconductor layer; (d) forming, on the heat diffusion suppressing layer, a second semiconductor layer composed of a Group III nitride semiconductor; and (e) irradiating the first semiconductor layer through the substrate with a light beam transmitted by the substrate and absorbed by the first semiconductor layer to decompose the first semiconductor layer.