PURPOSE: To easily and surely perform backwash treatment for recovering the function of a filter membrane, to automatize the treatment and to save labor by ozone-oxidizing, stripping off and removing adhesive substances such as suspended matter, when the backwash of the filter membrane for removing the suspended matter included in water flowing out from an activated carbon column is performed.

CONSTITUTION: An activated carbon column 7 provided with granulated activated carbon 8 into which ozonized water or nonozonized water is periodically introduced is installed, and also a filter 11 provided with a filter membrane 11a is arranged in the poststage. An ozonizer 3 for forming ozone gas, and a backwash water ozone reaction column 22 provided with a diffuser pipe 23 for diffusing the gaseous ozone into water as fine bubbles are installed. Suspended matter included in water flowing from the activated carbon column 7 is removed by a filter membrane 11a. And the filter membrane 11a is backwashed by using ozone water solution formed in the backwash water ozone reaction column 22 so that adhesive substances such as suspended matter entering the filter membrane 11a may be ozone-oxidized, stripped off and removed to recover the function of the filter membrane 11a.