PURPOSE: By utilizing the density differences among crude oil, crude oil sludge and water, a water layer is formed at the lowest part, and thereby crude oil sludge is prevented from adhering and solidfying to the bottom of an oil tank, and by giving the water in the water layer a turning motion, crude oil sludge is collected in the center of the turning flow and removed easily.

CONSTITUTION: Valve V12 is opened, tank 31 is filled with water, and water is supplied to crude oil tank 1 from crude oil receiving nozzles 22, 22' via valve V10, pump P4 and valve V5. The height of the water layer is such that the stirring vanes of stirrer 25 are immersed in water layer 4. In this condition, the tank with its contents is kept undisturbed. Then the most part of crude oil sludge sinks and condenses in the water layer at the bottom of the tank or in the boundary region. To remove crude oil sludge, the stirrer is actuated and a turning flow is produced. Since crude oil sludge piles up in the bottom center of tank 1, this can be easily removed.