PURPOSE: To prevent an ultrafiltration membrane from decrease of capacity for permeating water by feeding washing gas intermittently to sludge-contg. water while continuing supply of the sludge-contg. water to the ultrafiltration membrane.

CONSTITUTION: Sludge-contg. treated water from a bioreactor 1 for treating untreated water such as night soil with activated sludge, is separated into concentrated sludge and filtered water with a first ultrafilter 2. After admixing a coagulant to the filtered water, the mixture is separated into coagulated sludge and filtered water with a second ultrafilter 4. In this water treating apparatus, an air feeding device 6 for feeding pressurized air to a delivery side of each pressurizing pump P1, P2 in each feeding line R1, R2 to each ultrafilter 2, 4, is provided. In order to wash a filtration membrane 7 of the ultrafilter 2, 4, air is fed to the sludge-contg. water continuously for over a fixed time with a predetermined interval while continuing the supply of sludge-contg. water to each ultrafilter 2, 4 with the pumps P1, P2. The formation of turbulent flow is prompted by the gas on the surface of the filtration membrane 7, so the destruction of a cake layer or a gel layer on the surface of the membrane is caused effectively.