When a reflection plate is placed parallel at a suitable distance from a vibration plate with the stripes mode which radiates plane waves in the air, the acoustic load upon the vibration plate increases, and therefore electric power supplied to the ultrasonic source increases under a constant vibration amplitude. Making use of this relation, a rectangular vessel with a high intensity ultrasound field can be designed, the dimensions of which are easily determined by the wavelength of the sound wave and bending wave in the plate.