Affords Group III nitride semiconductor devices in which the leakage current from the Schottky electrode can be reduced. In a high electron mobility transistor 11, a supporting substrate 13 is composed of AlN, AlGaN, or GaN, specifically. An AlYGa1−YN epitaxial layer 15 has a full-width-at-half maximum of (0002) plane XRD of 150 sec or less. A GaN epitaxial layer 17 is provided between the gallium nitride supporting substrate and the AlYGa1−YN epitaxial layer (0