An apparatus for generating electric energy comprises a vibration plate, a supporting board, at least one side-wall unit and at least one piezoelectric substrate having a first- and a second end surfaces covered with a first- and a second electrodes, respectively. They all together form at least one cavity resonator. If a sound pressure from the outside arrives at the vibration plate, an acoustic vibration is excited in the vibration plate, and thereby a resonance vibration is induced in the cavity resonator. In this time, the piezoelectric substrate responds collectively to the resonance vibration. Thus, a resonance energy occurred in the cavity resonator is converted into an electric energy, which is delivered through the first- and second electrodes.