A composite substrate 10 is a substrate formed by bonding a piezoelectric substrate 12 and a support substrate 14 having a coefficient of thermal expansion lower than that of the piezoelectric substrate 12. The support substrate 14 has a first surface 14a bonded to the piezoelectric substrate 12 and a second surface 14b opposite to the first surface 14a. The coefficient of thermal expansion of the support substrate 14 is decreased along a thickness direction from the second surface 14b to an intermediate position 14c located between the first surface 14a and the second surface 14b.