A composite substrate 10 includes a sapphire body 1A, a seed crystal film 4 composed of gallium nitride crystal and provided on a surface of the sapphire body, and a gallium nitride crystal layer 7 grown on the seed crystal film 4 and having a thickness of 200 μm or smaller. Voids 5 are provided along an interface between the sapphire body 1A and the seed crystal film 4 in a void ratio of 4.5 to 12.5 percent.