Composite substrate for surface acoustic wave device, method of producing composite substrate for surface acoustic wave device, and surface acoustic wave device using composite substrate

Provided is a high-performance composite substrate for surface acoustic wave device which has good temperature characteristics and in which spurious caused by the reflection of a wave on a joined interface between a piezoelectric crystal film and a support substrate is reduced. The composite substrate for surface acoustic wave device includes: a piezoelectric single crystal substrate; and a support substrate, where, at a portion of a joined interface between the piezoelectric single crystal substrate and the support substrate, at least one of the piezoelectric single crystal substrate and the support substrate has an uneven structure, a ratio of an average length RSm of elements in a cross-sectional curve of the uneven structure to a wavelength λ of a surface acoustic wave when the substrate is used as a surface acoustic wave device is equal to or more than 0.2 and equal to or less than 7.0.