A method of removing nitrogen, in which the nitrogen is removed from wastewater containing a BOD component and ammonium nitrogen by anaerobic ammonium oxidation, comprising the steps of:distributing the wastewater into two portions which are a nitrification vessel and a denitrification vessel at a predetermined distribution ratio;mixing first treated water which is nitrified by ammonium oxidizing bacteria from ammonia to nitrite in the nitrification vessel, with second treated water which is denitrified by denitrification bacteria in the denitrification vessel; andfeeding the mixed water to an anaerobic ammonium oxidation vessel to simultaneously denitrify ammonia and nitrite by ammonium oxidizing bacteria, thereby obtaining a third treated water treated in the anaerobic ammonium oxidation vessel, which is then fed back to the denitrification vessel for denitrification and BOD decomposition.