A composite substrate includes a sapphire substrate and a layer of a nitride of a group 13 element provided on the sapphire substrate. The layer of the nitride of the group 13 element is composed of gallium nitride, aluminum nitride or gallium aluminum nitride. The composite substrate satisfies the following formulas (1), (2) and (3). A laser light is irradiated to the composite substrate from the side of the sapphire substrate to decompose crystal lattice structure at an interface between the sapphire substrate and the layer of the nitride of the group 13 element. 5.0≤(an average thickness (μm) of the layer of the nitride of the group 13 element/a diameter (mm) of the sapphire substrate)≤10.0 . . . (1); 0.1≤ a warpage (mm) of said composite substrate×(50/a diameter (mm) of said composite substrate)2≤0.6 . . . (2); 1.10≤a maximum value (μm) of a thickness of said layer of said nitride of said group 13 element/a minimum value (μm) of said thickness of said layer of said nitride of said group 13 element . . . (3)