<p>Disclosed is a three-medium boundary acoustic wave device (1) in which spurious responses caused by higher-order modes are inhibited, the boundary acoustic wave device (1) including: a first medium (2) composed of a piezoelectric element; a second medium (7); a third medium (6); and an IDT electrode (3). The first medium (2) propagates a fast transverse wave and a slow transverse wave. The second medium (7) is provided above the first medium (2). The third medium (6) is provided between the first medium (2) and the second medium (7). The sonic velocity of the transverse wave propagating through the third medium (6) is lower than the sonic velocity of the slow transverse wave propagating through the first medium (2) and is also lower than the sonic velocity of the transverse wave propagating through the second medium (7). The IDT electrode (3) is formed at the boundary between the first medium (2) and the third medium (6). The sonic velocity of the transverse wave propagating through the second medium (7) is lower than the sonic velocity of the fast transverse wave propagating through the first medium (2).</p>
