Provided is a technology capable of improving the quality of a GaN layer that is formed on an underlying substrate. A group III-nitride laminated substrate includes an underlying substrate, a first layer that is formed on the underlying substrate and is made of aluminum nitride, and a second layer that is formed on the first layer and is made of gallium nitride. The second layer has a thickness of 10 μm or less. A half-value width of (0002) diffraction determined through X-ray rocking curve analysis is 100 seconds or less, and a half-value width of (10-12) diffraction determined through X-ray rocking curve analysis is 200 seconds or less.