A boundary acoustic wave device includes an LiTaO3 piezoelectric substrate, a first dielectric medium layer disposed on the piezoelectric substrate, a second dielectric medium layer disposed on the first medium layer and having a sound velocity different from the first medium layer, and an interdigital electrode disposed at the boundary between the piezoelectric substrate and the first medium layer. The sound velocity of the first medium layer is less than the sound velocity of LiTaO3. The sound velocity of the second medium layer is greater than the sound velocity of LiTaO3. The inequality (h/λ)×a≦0.05 is satisfied, where H is the thickness of the first medium layer, h is the thickness of the interdigital electrode, λ is the period of electrode fingers of the interdigital electrode, and a is the ratio of the density of a metal of the interdigital electrode to the density of Au.