A washing method utilizing high-speed movement of microbubbles under ultrasound irradiation was studied. Air microbubble movement was observed under ultrasound irradiation of 20 kHz and 600 W with a highspeed camera. Microbubbles were observed to move at high velocities and cluster together. Copper plates with oil-based ink dirt or carbon dirt were prepared and used for the washing experiments. High-speed movement of microbubble clusters under ultrasound irradiation showed a cleaning effect with both samples. Spot damage by ultrasound cavitation of the sample surface did not occur in the presence of microbubbles, probably due to the covering of microbubble clusters near the sample surface. The results indicate that low-frequency (20 kHz) ultrasonic cleaning with microbubbles is suitable for weakly adhering dirt and for weak sample surfaces.