PURPOSE: To enable to effect a continuous filtration by a method wherein recovered waste oil is fed into a filter equipped with a vibrating screen when the oil is mixed so as to have a predetermined viscosity.

CONSTITUTION: Waste oil having a low viscosity and the same having a high viscosity are overflowed from underground pits 1, 2 and flowed into a central pit 3, then, are mixed by an agitator 4 so as to have an uniform quality. Subsequently, the mixed waste oil is fed into a regulating tank 7 and is mixed by an agitator 11 with light oil fed into the tank 7 by a pump 9 to make a viscosity of the mixed oil into 500 CP or less. Next, the waste oil regulated in a viscosity thereof is supplied to a filter 15 equipped with a vibrating screen 14. Filtrated sludge and water is proceeded sequentially and are thrown into a collecting can 17. When the mixed waste oil is regulated in the viscosity thereof and is supplied to the vibrating screen in such a manner, an effect similar to a back-washing device may be obtained, therefore, the filtrating portion may be kept clean all the times and a continuous filtration may be effected.