To eliminate the ncessity for discharging oils from a nuclear power station as waste oils as a rule, conduct the control and treatment of oils in a closed system, minimize the amt. of waste, enable water mixed into oils to be detected in the early stage while continuously removing the water, and thus reduce the amt. of waste oil.

An oil tank 1 for storing an oil such as a turbine oil is connected to a cleaning apparatus 2 in a closed circuit to clean the oil in the oil tank without taking it out. The cleaning apparatus 2 has a filter element comprising many sheets of paper and removes impurities in the oil in such a way that the oil is caused to pass through the lamination gaps between sheets of paper to thereby cause the impurities to be adhered to the surface of the paper.