Abstract: An activated sludge treatment process for waste water characterized in that, in the waste water treatment where waste water containing organic matter is introduced into an activated sludge reaction tank and the excess sludge formed in the reaction tank is introduced into a sludge digestion tank to effect the digestion of said excess sludge by intermittent aeration with air, a part of the digestion tank liquid is passed through a filtration apparatus having an ultrafiltration membrane, the filtrate passed therethrough is withdrawn outside the system, the remainder is circulated to the digestion tank to maintain the liquid volume of the digestion tank at a definite level, and when the amount of the reaction tank liquid introduced from the reaction tank into the digestion tank is taken as W.sub.1, the digestion tank liquid is returned in an amount of (0.2-0.8)W.sub.1 to the reaction tank, and thus the amount of excess sludge to be treated outside the system can be reduced or can be completely eliminated.