TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The bacteria are bacteria having a DNA sequence of 16S rRNA gene whose particular region contains at least one sequence chosen from any one of 9 fully defined 468 base pairs (SEQ ID No. 1-9) sequence given in the specification. In the method, an ammonium substrate and a nitrite substrate are fed into the cultivation tank, and a ratio of ammonium nitrogen to nitrite nitrogen fed into the cultivation tank is controlled in a range of 1:0.5-1:20. The feed constant (K) of the substrate is set in a range of 0.05-0.28, 0.06-0.15 or 0.07-0.12. The method further involves:(a) increasing stepwise the feed rate (Y') of the substrate to approach a rising curve represented by formula (I), and controlling a nitrite nitrogen concentration in the cultivation tank so that the nitrite nitrogen concentration is 70 mg/l or less;(b) after the bacteria enters the logarithmic growth phase, determining an actual consumption rate (y) of the substrate by comparing a first concentration of the substrate fed into the cultivation tank and a second concentration of the substrate remaining in liquid discharged from the cultivation tank;(c) calculating a consumption constant (k) for the consumption rate of the substrate using formula (II); and(d) setting the feed constant (K) to correspond to the calculated consumption constant (k).y=axexp (kxT),y=actual consumption rate of the substrate;a=substrate concentration at the start of cultivating; andk=consumption constant of the substrate.Preferred Equipment: The control device controls the ammonium feed device and the nitrite feed device so that a ratio of ammonium nitrogen to nitrite nitrogen fed into the cultivation tank is controlled in a range of 1:0.5-1:20. The equipment further comprises first measurement device for measuring a first concentration of each substrate fed into the cultivation tank, and second measurement device for measuring a second concentration of each substrate remaining in a liquid discharged from the cultivation tank, where the control device determines an actual consumption rate (y) of each substrate by comparing the first concentration of the substrate determined by the first measurement device and the second concentration of the substrate determined by the second measurement device.