NOVELTY - To-be-processed water (12) and activated sludge (14) are circulated within a processing tank (16) by aeration. Nitrification is carried out at the front stage of upward flow (28) of to-be-processed water. Dissolved oxygen of to-be-processed water is reduced by nitrification, and an anoxia region (32) is formed in the latter stage of the downflow (30) of to-be-processed water. In the anoxia region, to-be-processed water is denitrified, introducing new to-be-processed water (13) from the outside.    USE - Processing of wastewater.    ADVANTAGE - The method enables efficient processing of wastewater by using a single processing tank. Since to-be-processed water is circulated by aeration, a pump for aeration is not required. Hence, power consumption is reduced. Since an aerobic region and an anoxia region are simultaneously formed in the circulation path of to-be-processed water, new to-be-processed water can be introduced without inhibiting circulation by aeration. By setting the flow-path cross-section of downflow larger than that of upward flow, contact time with activated sludge is increased. Anoxia region can be easily formed in the latter stage of downflow. Non-uniformity of denitrification process is reduced. Denitrification can be carried out efficiently even when the service condition of processing apparatus changes. The processing apparatus is compact.    DETAILED DESCRIPTION - Wastewater obtained by filtering treated water (18) from to-be-processed water is processed as follows. To-be-processed water is stored with activated sludge in a processing tank provided with a filtration membrane (20), and aeration of to-be-processed water is carried out from the lower portion of the film surface of the filtration membrane. To-be-processed water and activated sludge are circulated within the processing tank by aeration. Nitrification is carried out at the front stage of upward flow of to-be-processed water. Dissolved oxygen of to-be-processed water is reduced by nitrification, and an anoxia region is formed in the latter stage of the downflow of to-be-processed water. In the anoxia region, to-be-processed water is denitrified, introducing new to-be-processed water from the outside. An INDEPENDENT CLAIM is included for wastewater processing apparatus (10).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the wastewater processing apparatus.    Wastewater processing apparatus (10)    To-be-processed water (12)    New to-be-processed water (13)    Activated sludge (14)    Processing tank (16)    Treated water (18)    Filtration membrane (20)    Upward flow (28)    Downflow (30)    Anoxia region (32)