PURPOSE: To separately filter service water and treated water, to supply good- quality water and to facilitate the administrative and maintenance.

CONSTITUTION: Service water is filtered and stored in a water storage tank 14, the treated water in the tank 14 is circulated by a primary circulating line 15 connected to the tank 14, and the residual chlorine concn. in the treated water is detected and kept at a specified value. The obtained good-quality water is then circulated by a secondary circulating line 20 connected to the tank 14 and filtered by a hollow-fiber membrane at each point 22 of use connected to the secondary circulating line 20.