A boundary acoustic wave device includes a piezoelectric substance made of LiNbO3 having a surface obtained by rotating a Y axis by about 15°±10° as a primary surface, a dielectric substance which is made of a silicon oxide and which is laminated to the piezoelectric substance, and an electrode structure disposed at a boundary between the piezoelectric substance and the dielectric substance and which includes an IDT arranged to utilize a boundary acoustic wave propagating along the boundary. When the density of the IDT, the thickness thereof, the wavelength deter-mined by the period of electrode fingers of the IDT, and the duty ratio thereof are represented by ρ (kg/m3), H (μm), λ (μm), and x, respectively, x and the product of H/λ and ρ are set in a range that satisfies the following formula (1): (H/λ)×ρ>70.7924(x+0.055)(−2.884)+797.09  Formula (1).