A Lamb-wave resonator includes: a piezoelectric substrate; an IDT electrode disposed on one principal surface of the piezoelectric substrate, having bus bar electrodes each connecting one ends of a plurality of electrode finger elements, the other ends of the plurality of electrode finger elements being interdigitated with each other to form an apposition area; and a pair of reflectors disposed on the one principal surface of the piezoelectric substrate, and respectively arranged on both sides of the IDT electrode in a propagation direction of a Lamb wave, wherein denoting a thickness of the piezoelectric substrate in the apposition area of the electrode finger elements as ti, a thickness of the piezoelectric substrate in areas between respective ends of the apposition area in a direction perpendicular to the propagation direction of the Lamb wave and the bus bar electrodes as tg, and a wavelength of the Lamb wave as λ, the thickness ti exists in a range represented by 0