A method for manufacturing a composite piezoelectric substrate in which a piezoelectric substrate and a supporting substrate are prepared, ions are implanted in the piezoelectric substrate to form a defective layer at a predetermined depth in the piezoelectric substrate, impurities that are adhered to a surface of the piezoelectric substrate or a surface of the supporting substrate are removed to expose the constituent atoms thereof and to activate the surfaces, the supporting substrate is bonded to the piezoelectric substrate to form a bonded substrate body, the bonded substrate body is separated at the defective layer so that a separation layer between the surface of the piezoelectric substrate and the defective layer is separated from the piezoelectric substrate and bonded to the supporting substrate to form a composite piezoelectric substrate, and the surface of the separation layer of the composite piezoelectric substrate is smoothed.