PURPOSE: To eliminate NH4-N and NOX-N at the same time with high efficiency, by carrying out a biological treatment of waste water contg. nitrogen in high concentration without dilution or by very low dilution using an aeration apparatus having ≥20% dissolving efficiency of oxygen.

CONSTITUTION: In a reaction tank 10 for treating waste iquid contg. nitrogen in high concentration beyond the suitable range of the ordinary activated sludge treatment directly without dilution or by very low dilution, an aeration equipment 5 having ≥20% dissolving efficiency of oxygen is applied to supply air 8 for aeration in very fine bubbles into the liquid mixture. By setting up the supplying quantity of the air for aeration within the suitable range, both functions for removing NH4-N and NOX-N can be allowed to coexits with high efficiency. The above-mentioned suitable range is controllable by proper management of the dissolved oxygen value and the oxidation-reduction potential value of the liquid mixture.