PURPOSE: To achieve a long life of a membrane and to prevent damage thereto by a method wherein a filtration membrane is constituted of a hydrophilic material and a pneumatic pressure is applied to a liquid passed through the membrane and held at a filtration membrane loading part through a liquid outlet port so that the filtration membrane can be surely cleaned and a pneumatic pressure required when the membrane is cleaned can be reduced.

CONSTITUTION: During a filtration operation, a flow line 7 and flow line 8 are switched by a line switching three-way valve 6 so that both the valves communicate with each other. As a result, a liquid which is filtrated by a hollow fiber membrane module 100 and filled in a hollow fiber membrane bundle loading part 106 flows out of a liquid outlet port 109. And in the case of backward washing, a flow line 9 is brought into communication with the line 7 by means of the valve 6. Consequently compressed air pressure is applied onto the surface of the liquid filled in the part 106 of the module 100 so that the liquid is discharged through a raw liquid inlet port 7, whereby foreign matters attached and accumulated on the raw liquid side (inner side) of membrane wall of a hollow fiber membrane 102 are removed so that the membrane 102 is cleaned.