A composite substrate is a plate-shaped body where a first substrate and a second substrate overlap. The composite substrate includes a hole in the first substrate and the second substrate in a thickness direction of the plate-shaped body. A ratio A/B of an average thickness A of the first substrate to an average thickness B of the second substrate is ⅕ or less. An interface between the first substrate and the second substrate on an inner wall of the plate-shaped body that is positioned at the hole includes a part that is covered by a covering layer that contains a component that composes the second substrate.