A Group III nitride semiconductor substrate is formed of a Group III nitride single crystal, and has a diameter of not less than 25.4 mm and a thickness of not less than 150 μm. The substrate satisfies that a ratio of Δα/α is not more than 0.1, where α is a thermal expansion coefficient calculated from a temperature change in outside dimension of the substrate, and Δα is a difference (α−αL) between the thermal expansion coefficient α and a thermal expansion coefficient αL calculated from a temperature change in lattice constant of the substrate.