A compound semiconductor substrate includes a first substrate and a second substrate made of single crystal gallium nitride. In each of the first substrate and the second substrate, one surface is a (0001) Ga-face and an opposite surface is a (000-1) N-face. The first substrate and the second substrate are bonded to each other in a state where the (000-1) N-face of the first substrate and the (000-1) N-face of the second substrate face each other, and the (0001) Ga-face of the first substrate and the (0001) Ga-face of the second substrate are exposed.