An apparatus for removing nitrogen is provided herein. In the apparatus, the nitrogen is removed from wastewater containing a BOD component and ammonium nitrogen by anaerobic ammonium oxidation. The apparatus includes a distributor, which distributes the wastewater into two portions; a nitrification vessel which connects to a first portion of said two portions, in which ammonia contained in wastewater distributed to the first portion is nitrified by ammonium oxidizing bacteria; a denitrification vessel which connects to a second portion of said two portions, in which wastewater distributed to the second portion is denitrified by denitrification bacteria; an anaerobic ammonium oxidation vessel, in which mixed water of first treated water from the nitrification vessel and second treated water from the denitrification vessel is subjected to anaerobic ammonium oxidation by ammonium oxidizing bacteria; and a returning line which feeds back a third treated water from the anaerobic ammonium oxidation vessel to the denitrification vessel for denitrification and BOD decomposition.