PURPOSE: To effect the washing of granules in high efficiency, by mixing a washing liquid with granules to be washed and applying an impulse to the washing liquid in the mixed state, thereby generating ultrasonic vibration suitable for the washing of the granules by the generation and collapse of bubbles.

CONSTITUTION: Granules 22 to be washed are mixed with a washing liquid and the mixture is ejected into a concave space 14 of an ultrasonic generation vessel 13. Bubbles are formed by impulse wave generated in the concave space 14 and the bubble is broken into a number of small bubbles by the collision of the following washing liquid. The energy of the ultrasonic vibration generated in the ultrasonic generation vessel 13 is amplified by the reverberation effect in the concave space 14.