Bulk acoustic resonator and filter device

A bulk acoustic resonator includes: a substrate; a first electrode disposed on the substrate; a piezoelectric layer disposed to cover at least a portion of the first electrode; a second electrode disposed to cover at least a portion of the piezoelectric layer; and an insertion layer disposed below a partial region of the piezoelectric layer. A thickness of the first electrode in an active region in which the first electrode, the piezoelectric layer, and the second electrode overlap one another is less than a thickness of a region outside the active region. An angle of inclination of an internal side surface of the insertion layer is different from an angle of inclination of an external side surface of the insertion layer.