A large amount of heavy fuel oil C was spilled out to the sea in the wreck of Russian tanker Nakhodka in January, 1997, and serious damage was brought to the coasts which face the Sea of Japan. As the cleaning cost of the oil recovery vessel used in such a oil spill case is very high, the cost reduction is requested. On the other hand, oil recovery on the coast needs enormous human power. Therefore,it is necessary to develop a cheaning technology of oil that has small environmental impact and high efficiency.
　We carried out the oil cleaning experiments which uses only physical power of ultrasonic waves and no detergent etc. They were executed on the heavy fuel oil adhered iron plates, the heavy fuel oil adhered gravel, and the heavy fuel oil mixed sand.
　We obtained relation between the cleaning time and frequency or power density of ultrasonic waves. The cleaning experiments carried out for the heavy fuel oil adhered iron plate and the heavy fuel oil adhered gravel.
　The heavy fuel oil which adhered to the iron plate is changed into particles by the action of the ultrasonic waves,and they are dispersed in the cleaning water. They adhered to the iron plate again Therefore, it is impossible to remove the oil completely. However, about 80％ of the heavy fuel oil which adhered to the iron plate was able to be removed.
　Heavy fuel oil mixed sand was cleaned almost completely. The difference of the effect of cleaning according to the frequency of the ultrasonic waves was not clear.
　Cleaning effect of a certain level is verified through these experiments only with physical power proceeding from the ultrasonic waves.