To provide a boundary acoustic wave device which is compact and excellent in temperature stability, possesses excellent Q-value, and features low cost.

A boundary acoustic wave device is obtained by patterning a comb electrode with a film thickness of hm and an electrode finger period of , whose main ingredient is aluminum, and a shorting reflector (thickness hr) onto the surface of a YX-LN single crystalline piezoelectric substrate, and then by forming a silicon oxide film with a film thickness of h1 and an aluminum nitride film 6 with a film thickness of h2 on the comb electrode and the reflector, wherein: 2.5hr/8.5%.