An optimum design procedure of the vibrating system for a Multi-Pinhole-Plate ultrasonic atomizer has been studied. It is composed of a thin plate with many pinholes and a circular ring piezoelectric element. The greatest efficiency was achieved at under two conditions. One was the coincidence among the first resonance frequencies of the inner portion inside the element and the outer portion and the second resonance frequency of the entire system. The other was when the inner diameter of the element was equal to that of the second nodal circle of a circular plate.