An acoustic boundary wave device includes a piezoelectric body, an IDT layer formed on the piezoelectric body, a pad electrode layer formed on the piezoelectric body and connected to the IDT layer, a first dielectric layer formed on the piezoelectric body and covering at least a part of the IDT electrode layer, and a second dielectric layer formed on the piezoelectric body, covering the first dielectric layer, and having an opening through which at least a part of a top face of the pad electrode layer is exposed. The metal forming lateral faces of the pad electrode layer diffuses more readily into the first dielectric layer than into the second dielectric layer. The second dielectric layer covers the lateral faces of the pad electrode layer and prevents the first dielectric layer from touching the lateral faces of the pad electrode layer.