A production method for a composite substrate according to the present invention comprises (a) a step of mirror-polishing a piezoelectric-substrate side of a laminated substrate formed by bonding a piezoelectric substrate and a support substrate; (b) a step of performing machining using an ion beam or a neutral atom beam so that a thickness of an outer peripheral portion of the piezoelectric substrate is larger than a thickness of an inner peripheral portion and a difference between a largest thickness and a smallest thickness of the inner peripheral portion of the piezoelectric substrate is 100 nm or less over an entire surface; and (c) a step of flattening the entire surface of the piezoelectric substrate to remove at least a part of an altered layer formed by the machining using the ion beam or the neutral atom beam in the step (b).