A composite substrate has: a first substrate having a first bump protruding therefrom; and a second substrate having a first surface in contact with the first bump, and a second surface opposite to the first surface, the second surface having a second bump protruding therefrom, the second substrate being laminated on the first substrate in a thickness direction perpendicular to the first surface. The first bump and the second bump are partially overlapped with each other as viewed in the thickness direction. The second bump has a rigidity which is lower than a rigidity of the first bump.