A filtering device includes a container having an inlet for introducing liquid to be treated, an annular multi-layered prepacked screen assembly provided in the container and having an outlet for treated liquid. The screen assembly includes three or more cylindrical screens having different diameters and being coexially disposed in the container with their axes extending vertically in the container. An annulus is formed between the respective cylindrical screens. Two or more different filter materials performing different filtering functions are filled one in each of the annuluses between the cylindrical screens.