In a boundary acoustic wave resonator, apodization weighting is performed on an IDT electrode so that an intersecting width decreases as a location moves outward in a boundary acoustic wave propagation direction. An inner side of a first busbar includes inclined portions that are disposed a predetermined distance from an envelope portion B1 on the side of the first busbar. An inner side of a second busbar includes inclined portions that are disposed a predetermined distance from an envelope portion on the side of second busbar. The sum of the numbers of electrode fingers and dummy electrodes that a straight line extending from the end of at least one of electrode fingers in the boundary acoustic wave propagation direction crosses before reaching the side of the first busbar or the side of the second busbar is equal to or greater than 5.