A boundary acoustic wave device includes a piezoelectric substrate made of single-crystalline LiTaO3, a first medium layer disposed on the piezoelectric substrate and made of a dielectric, a second medium layer disposed on the first medium layer and made of a dielectric having a sound velocity different from that of the first medium layer, and at least one 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.