PROBLEM TO BE SOLVED: To provide a fixed bed type biological denitrification method capable of efficiently exhausting a nitrogen gas stored in a carrier packed bed while preventing the destruction of anaerobic conditions and the generation of an exhaust gas involving bad smell.SOLUTION: Provided is a method of subjecting oxidation state nitrogen-containing water to denitrification treatment at a fixed bed type denitrification part having a packed bed filled with a biofouling carrier, in which a nitrogen gas generated by denitrification reaction in the packed bed and made to flow out from the packed bed is poured into the lower part of the packed bed, thus the gas stored at the inside of the packed bed is pulled out from the packed bed.
