PURPOSE: To drastically increase filtering treatment capacity by a method wherein the filter elements provided in the second chamber are provided on upper quadruplex circles and the second backwashing nozzles are constituted as a pair of the nozzles corresponding to two sets of the filter elements, arranged on two circles and two sets of the filter elements arranged on two remaining circles.

CONSTITUTION: A large number of cylindrical filter elements 31a..., 31b..., 31c..., 31d... are provided in the second chamber 30 so as to be respectively arranged on the circumferences of quadruplex circles round an axial line (l) at a predetermined interval. The first backwashing nozzles are respectively provided to the leading end parts of backwashing pipes 47a, 47b, and one of them has the revolving radius corresponding to the filter elements 31a... arranged on the outermost periphery and the other one of them has the revolving radius corresponding to the filter elements 31d... arranged on the innermost periphery. The second backwashing nozzles are provided to the leading end parts of backwashing pipes 50a, 50b and one of them has the revolving redius corresponding to the filter elements 31b... arranged on the second circumference of a circle from the outermost periphery, and the other one of them has the revolving radius corresponding to the filter elements 31c... arranged on the third circumference of a circle from the outermost periphery.