The composite substrate of the present disclosure has a piezoelectric substrate having a first surface and a second surface opposing the first surface, a support substrate having a third surface in contact with the second surface and a fourth surface opposing the third surface, and a through hole penetrating from the first surface to the fourth surface. The through hole has a tapered shape having a diameter decreasing from the first surface to the fourth surface and has a stepped surface having a decreasing diameter in the support substrate. The piezoelectric device of the present disclosure comprises the composite substrate and a conductor located within the through hole.
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