An object is to provide a film bulk acoustic resonator capable of improving resonant characteristics by reducing the generation of a standing wave to be caused by a transverse-mode acoustic wave to a minimum. In a film bulk acoustic resonator including a resonant portion A having a piezoelectric material layer 3 sandwiched between a first electrode 2 and a second electrode 4, the resonant portion A is configured to have a planar shape that is an ellipse having a part thereof cut off along a straight line L. The straight line L intersects at least one of a minor axis and a major axis of the ellipse, and preferably intersects both the minor axis and the major axis, and passes through the center of the ellipse.