NOVELTY - A support (26) containing nitrous-acid-type-nitrification microbe, or aerobe which decomposes soluble nitrogen and nitrous-acid-type-nitrification microbe, and anaerobic ammonia oxidation microbe, is fluidized in a reaction tank (14). To-be-processed water containing soluble nitrogen is flown into the reaction tank and denitrification is carried. The support injection rate is adjusted such that production amount of nitrous acid is set to a level to the extent that nitric acid type nitrification reaction is suppressed.    USE - Biological nitrogen removal in water treatment systems of to-be-process water containing soluble nitrogen (claimed) e.g. city sewage and household wastewater.    ADVANTAGE - The method achieves simultaneous acceleration of nitrous-acid-type nitrification reaction and suppression of nitric-acid-type nitrification reaction. Anaerobic ammonia oxidation reaction is utilized relieving the restrictions with respect to the ammonia nitrogen concentration of to-be-processed water.    DETAILED DESCRIPTION - A support which carries, in the surface portion, a two-layered microbial film where nitrous-acid-type-nitrification microbe contributing to nitrous-acid-type-nitrification reaction, or aerobe which decomposes soluble nitrogen other than ammonia into ammonia and nitrous-acid-type-nitrification microbe exist at outer side as dominant species, and anaerobic ammonia oxidation microbe contributing to anaerobic ammonia oxidation reaction exists inside in a form surrounded by the nitrous-acid-type-nitrification microbe as dominant species, is fluidized in a reaction tank. To-be-processed water containing soluble nitrogen is flown into the reaction tank under aerobic conditions and denitrification is carried out by anaerobic ammonia oxidation reaction. The support injection rate (support total surface area per unit volume of reaction tank) is adjusted such that the production amount of nitrous acid by nitrous-acid-type-nitrification reaction is set to a level to the extent that nitric acid type nitrification reaction is suppressed. INDEPENDENT CLAIMS are included for the following:    (1) biological nitrogen removal apparatus;    (2) water treatment system (10); and    (3) support.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the water treatment system. (Drawing includes non-English language text)    Water treatment system (10)    Sedimentation tank (12)    Reaction tank (14)    Support (26)    Partition (30)