Abstract: A bulk-acoustic wave (BAW) resonator includes a central portion in which a first electrode, a piezoelectric layer, and a second electrode are sequentially stacked on a substrate, and an extension portion extending externally from the central portion, and an insertion layer and a loss prevention film are disposed in the extension portion between the substrate and the second electrode. The loss prevention film is formed to have a thickness of 50 ? to 500 ?. The insertion layer is stacked on the loss prevention film, and has a side surface opposing the central portion, the side surface is formed as a first inclined surface having a first inclination angle. The loss prevention film has a side surface opposing the central portion, the side surface is formed as a second inclined surface having a second inclination angle. The second inclination angle is formed to be greater than the first inclination angle.