The composite substrate is a substrate used to manufacture an acoustic wave device, and includes a support substrate, a piezoelectric substrate, and a adhesive layer with which the support substrate and the piezoelectric substrate are bonded to each other. In the composite substrate, assuming that a surface of the piezoelectric substrate that is bonded to the support substrate is defined as a first surface and a surface at the side opposite to the first surface is defined as a second surface, the piezoelectric substrate is formed such that the first surface is inside the second surface when the first surface is projected onto the second surface in a direction perpendicular to the second surface. In other words, the composite substrate has an outer peripheral surface that is formed such that the circumference thereof increases toward the top surface of the piezoelectric substrate.