An elastic boundary wave apparatus that is capable of suppressing an undesired response due to a comb-shaped capacitor electrode includes a first medium; a second medium stacked on the first medium; and a plurality of electrodes arranged in the boundary between the first and second media, wherein a filter and/or a resonator, and an electrostatic capacitor that is electrically connected to the filter and/or the resonator are defined by the plurality of electrodes, the filter and/or the resonator each include an IDT electrode, and the electrostatic capacitor is defined by a comb-shaped capacitor electrode. When the period of the electrode fingers of the IDT is denoted as .lamda.m, the sound velocity of an elastic boundary wave in the IDT is denoted as Vm, the period of the electrode fingers of the comb-shaped capacitor electrode is denoted as .lamda.c, and the sound velocity of a transverse wave having a lower sound velocity among the sound velocities of the slower transverse waves through the first and second media in the normal direction of the electrode fingers of the comb-shaped capacitor electrode is denoted as Vc, the relationship Vm/.lamda.m