An epitaxial substrate including a single-crystal base material and an upper layer of a group III nitride crystal film which is epitaxially formed on a main surface of the base material undergoes heating treatment in a nitrogen atmosphere at 1950° C. or higher for one minute. The result showed that, while a γ-ALON layer was formed only at the interface between the base material and the upper layer, the dislocation density in the group III nitride crystal was reduced to one tenth or less of the dislocation density before the heating treatment. The result also showed that the surface of the epitaxial substrate after the heating treatment had a reduced number of pits, which confirmed that high-temperature and short-time heating treatment was effective at improving the crystal quality and surface flatness of the group III nitride crystal.