A bonded body includes a supporting substrate; a piezoelectric material substrate composed of a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and a bonding layer bonding the supporting substrate and piezoelectric material substrate and contacting a main surface of the piezoelectric material substrate. At least one of a bonding surface of the supporting substrate and a bonding surface of the piezoelectric material substrate, as measured by spectral ellipsometry with Δ being assigned to a difference of phases of p-polarized light and s-polarized light of a reflected light, has a difference of the maximum and minimum values of the difference Δ of the phases in a wavelength range of 400 nm to 760 nm of 70° or lower.