Boundary acoustic wave devices are both compact and possess excellent temperature stability. Yet these devices have the drawback that the Q value cannot be raised, and a high cost thin-film technology is required. This invention provides a boundary acoustic wave device possessing excellent Q value along with a low cost. A boundary acoustic wave device including a film whose main ingredient is aluminum at a thickness hm, and a shorting reflector (thickness hr) and a IDT with an electrode finger period of lambda, are patterned onto the surface of a theta YX-LN single crystalline piezoelectric substrate; and a silicon oxide film with a thickness h1 and an aluminum nitride film 6 with a thickness h2 are formed on that comb electrode and reflector, wherein: 2.5≦hr/λ≦8.5% is obtained.