Disclosed is a method and an apparatus for disinfecting by adding a disinfectant having excellent handleability to wastewater containing ammonia nitrogen. SOLUTION: Immediately after dissolving a predetermined amount of a solid hypochlorous acid source (B) in a drug (A) containing bromine ions to prepare a disinfectant containing hypochlorous acid, the disinfectant is treated with an ammoniacal disinfectant. Disinfection method to be added to nitrogen-containing wastewater, mixing section 1 for adding and mixing solid hypochlorous acid source (B) to drug (A), receiving mixture from mixing section 1 and converting the mixture to drug (A) Disinfectant preparation unit 2 for dissolving chloric acid source (B) to prepare disinfectant, first line 3 for supplying drug (A) to disinfectant preparation unit 2, disinfectant immediately after preparation from disinfectant preparation unit 2 A disinfecting apparatus comprising a disinfectant supply line 4 for supplying wastewater containing ammoniacal nitrogen to wastewater (hereinafter also referred to as “water to be treated”). [Selection diagram] Fig. 1【解決課題】取り扱い性に優れた消毒薬をアンモニア性窒素含有排水に添加して、消毒する方法及び装置を提供する。【解決手段】臭素イオンを含む薬剤（A）に所定量の固体の次亜塩素酸源（B）を溶解させて、次亜塩素酸を含む消毒薬を調製した直後に、消毒薬をアンモニア性窒素含有排水に添加する消毒方法、及び薬剤（A）に固体の次亜塩素酸源（B）を添加して混合させる混合部１、混合部１から混合物を受入れて薬剤（A）に次亜塩素酸源（B）を溶解させて消毒薬を調製する消毒薬調製部２、消毒薬調製部２に薬剤（A）を供給する第１ライン３、消毒薬調製部２から調製直後の消毒薬をアンモニア性窒素含有排水（以下「被処理水」ともいう。）に供給する消毒薬供給ライン４を具備する消毒装置。【選択図】図１
