A manufacturing method for a composite substrate includes a step of forming an intermediate layer on a support substrate, a step of irradiating, with a fast atom beam, each of a surface of a piezoelectric material substrate and a surface of the intermediate layer formed on the support substrate, a step of further irradiating the surface of the piezoelectric material substrate with the fast atom beam, and forming a sputtered film containing the material of the piezoelectric material substrate on the surface of the intermediate layer, a step of obtaining a bonded body having a waveguide substrate by bonding the piezoelectric material substrate and the intermediate layer on which the sputtered film is formed, and a step of irradiating this bonded body with laser light in a surface direction of the composite substrate, wherein the composite substrate is formed using the bonded body irradiated with the laser light.