PURPOSE: To provide safekeeping for a porous tube filter by passing a dehumidifying gas through the filter to keep the filter in a dry state during the suspension of operation.

CONSTITUTION: On-off valves 10V, 11V, and 12V are respectively provided to a supply pipe 10 which supplies a waste water from a circulating pump 8 to an inlet chamber 3, a return pipe 11 which returns a concentrated waste water from an outlet chamber 5 to a circulating tank 7, and a recovery pipe 12 which extracts a treated water from filtrate chamber 9 to the outside. Further, water discharging pipes 13, 14 and gas discharging pipes 15, 16 each having on-off valves are provided to the bottoms of the chambers 3 and 9, and gas supply pipes 17 and 18 each having on-off valves are provided to the chambers 5 and 9, and a dehumidified gas is then supplied to the pipes 17, 18 by compressing it. Thus the drying of the inner surface, outer surface, and pipe wall of the filter is performed in sequence by changing the opening and closing of the valves 15V, 16V and the valves 17V, 18V for every fixed time. As a result, the propagation of microorganism in the filter is restricted, and the clogging of the filter is prevented.