Raw water containing ammonium-nitrogen is introduced into a nitrification tank and nitrification of nitrite-nitrogen by nitrite oxidizing bacteria is inhibited according to inhibition of the ammonium-nitrogen, thereby conducting stable nitrification of nitrous acid type at a high load by the action of ammonium oxidizing bacteria. Carbonate and/or bicarbonate are added into the nitrification tank 1. The inorganic carbon concentration in the nitrification tank 1 is maintained at 35 mg-C/L or more. The nitrified liquid thus obtained is introduced into a denitrification tank and denitrification reaction is conducted by the action of denitrifying bacteria with the remaining ammonium-nitrogen as an electron donor and the nitrite-nitrogen as an electron acceptor.