In a boundary acoustic wave device, a silicon oxide layer and a dielectric layer exhibiting a higher acoustic velocity than the silicon oxide layer are stacked on a LiTaO3 piezoelectric substrate, and IDT electrodes are disposed in grooves in an upper surface of the piezoelectric substrate. Each of the IDT electrodes includes a main electrode layer made of a metal having a density of about 16 g/cm3 or higher, and an auxiliary electrode layer made of a metal having a density within a range of about 3 g/cm3 to about 16 g/cm3. Further, θ of Euler angles (0°, θ, 0°) of the piezoelectric substrate is set within a range of about (θ0−20°) to about (θ0+20°) with respect to θ0 expressed by the formula: θ0=65.53G+0.7568F+0.8454E+7.091D+1.609C−0.03789B−3.535A+60.85.