Abstract: A method for incubation of nitrifying bacteria in a high concentration which are contained in small amount in activated sludge includes subjecting the activated sludge to nitrification and acclimation culture for about one to two months using a sludge-treated waste liquid such as dehydrate sludge filtrate and digested eluate under such a condition that the dissolved oxygen is 2-4 mg/liter, the pH is 7.5-8.5 and the temperature is 25-35° C. and, at the same time, constantly maintaining the pH within the pH range of 7.7-8.5 by addition of an incubation promoter comprising a mixture of sodium carbonate and sodium hydrogen carbonate.