PURPOSE: To separate oil from a settling sludge, to deposit high solid content oil in the bottom of a tank, and to facilitate after-treatment of the sludge, by charging the oil to be stored into a tank wherein water is fed into the bottom part.

CONSTITUTION: The tank 1 is charged with the water 2 in the bottom part into which the oil 3 is poured. An oil component in the sludge 4 is extracted with water to rise to the surface of the water 4 while the sludge 4 being settled in the water 2 and united with the oil 3. The said sludge 4 is thus deposited in the bottom of the tank 1. Since the energy required for sludge suspension and oil extraction is reduced, the operation effciency for sludge removal is increased. The amount of hydrocarbons produced in the tank 1 is decreased and the tank 1 can be cleaned safely in a short time.