To economically remove nitrogen in the biological denitrifying stage contg. activated sludge by packing and holding a bent small rod-shaped packing consisting of a floatable material at a specified rate to form a floating packed bed and passing the activated sludge from the sewage nitrating stage and the return sludge from a settling stage.

A sewage 1 is introduced into a denitrification part A along with return sludge 2, and an activated sludge 3 (nitrated slurry) contg. nitrate nitrogen is circulated to the denitrification part A from a nitration tank B by a circulating pump 12. A packing consisting of a small bent expanded polystyrene rod is packed in a denitrification part A to form a floating packing bed having ≥30% void ratio. The nitrate nitrogen contained in the activated sludge 3 is removed by the denitrifying bacteria depositing on the packing, the effluent slurry 5 is introduced into a nitration tank B, and the ammoniacal nitrogen in the sewage is nitrated by nitrating bacteria under aerobic conditions. The activated sludge 11 discharged from the nitration part B is sent to a settling tank 7 and settled to be separated.