A boundary acoustic wave device is provided which can decrease insertion loss regardless of whether the duty ratio is high or low. In a boundary acoustic wave device (1) including: a piezoelectric substance (2) composed of LiNbO 3 having a surface obtained by rotating a Y axis by 15°±10° as a primary surface; a dielectric substance (3) which is composed of a silicon oxide and which is laminated to the piezoelectric substance (2); and an electrode structure (4) which is disposed at a boundary between the piezoelectric substance (2) and the dielectric substance (3) and which includes an IDT to utilize a boundary acoustic wave propagating along the boundary, when the density of the IDT, the thickness thereof, the wavelength determined by the period of electrode fingers of the IDT, and the duty ratio thereof are represented by ρ (kg/m 3 ), H (µm), λ (µm), and x, respectively, x and the product of H/λ and ρ are set in the range that satisfies the following formula (1). Mathematical Expression 1 H / λ × ρ > 70.7924 ⁢ x + 0.055 - 2.0884 + 797.09