Abstract: A method for operating an anaerobic ammonium oxidation vessel in anaerobic ammonium oxidation in which ammonium and nitrite in water to be treated are simultaneously denitrified by anaerobic ammonium oxidizing bacteria, the method comprising the steps of: withdrawing a part of the anaerobic ammonium oxidizing bacteria from one anaerobic ammonium oxidation vessel in which the anaerobic ammonium oxidizing bacteria have been acclimatized, and introducing the withdrawn anaerobic ammonium oxidizing bacteria into another anaerobic ammonium oxidation vessel in which acclimatization is to be carried out to perform start-up operation, wherein the anaerobic ammonium oxidizing bacteria having been acclimatized in the one anaerobic ammonium oxidation vessel are provided as microbe-immobilized materials in which the bacteria are attached to and immobilized on immobilizing materials or entrapped and immobilized in immobilizing materials.