NOVELTY - Nitrogen of nitrogen-containing wastewater is oxidized with nitrifying bacteria and nitrate nitrogen and/or nitrite nitrogen is produced. The nitrate nitrogen and/or nitrate nitrogen is reduced to nitrogen with denitrifying bacteria. Remaining hydrogen donor is decomposed by aerobic microbe in reaeration process, and processed water is obtained. The to-be-processed water is supplied through porous flow carrier (103) having hole (I) to hold denitrifying bacteria and porous flow carrier (113) having hole (II) to hold protozoa which preys on denitrifying bacteria.    USE - For processing of nitrogen-containing wastewater.    ADVANTAGE - The nitrogen-containing wastewater is processed efficiently, economically and steadily in a compact processing apparatus.    DETAILED DESCRIPTION - Ammonia nitrogen and/or organic nitrogen contained in nitrogen-containing wastewater is oxidized with nitrifying bacteria and nitrate nitrogen and/or nitrite nitrogen is produced. A hydrogen donor is added to treated water of nitrifying process, and nitrate nitrogen and/or nitrate nitrogen is reduced to nitrogen with denitrifying bacteria. The hydrogen donor remaining in the denitrifying process is decomposed by aerobic microbe in reaeration process, and processed water is obtained. The to-be-processed water is supplied through a porous flow carrier (103) having hole (I) to hold denitrifying bacteria and which prevents penetration of bacteria into deep area of flow carrier and a porous flow carrier (113) having hole (II) to hold protozoa which preys on denitrifying bacteria, in the reaeration process. The hole (II) has larger diameter than hole (I). An INDEPENDENT CLAIM is included for processing apparatus of nitrogen-containing wastewater.    DESCRIPTION OF DRAWING(S) - The figure shows the outline of the wastewater treatment plant. (Drawing includes non-English text)    Wastewater treatment plant (10)    Nitrifying tank (101)    Flow carrier (103,113)    Denitrifying tank (111)    Aeration apparatus (125)