NOVELTY - The tank (1) has an inner tank (3) divided into upper and lower portions (3c,3d) used as aerobic and anaerobic microorganism processing tanks. A control apparatus (4) controls circulation ratio of to-be-processed water by opening and closing valve based on measured value from hydrogen ion concentration and dissolved oxygen amount of to-be-processed water or by vertical motion of control board (5), such that the anaerobic and aerobic microbial treatments are continuously made by supplying natural water from supply port (10) and by circulating raw water and sludge through tank.    USE - Microorganism reaction tank for sludge treatment of waste water in foodstuff manufacturing plant used for manufacturing cake and factory for manufacturing international goods.    ADVANTAGE - The anaerobic and aerobic microbial treatments are continuously made by supplying natural water from supply port and by circulating raw water and activated sludge through tank, so that the sludge is prevented from carrying out sedimentation fixation in lower portion and anaerobic-aerobic microbial treatment of can be performed continuously and effectively. Since the sedimentation fixed prevention apparatus is provided, the capacity of reaction tank becomes large and installation of microorganism reaction tank is easy.    DETAILED DESCRIPTION - The stirring apparatus is arranged for stirring processing tanks. The sedimentation fixed prevention apparatus (12) is arranged to prevent sedimentation fixation of sludge made to settle. The control board is provided in the top outer periphery of cylindrical inner tank to settle the sludge settle. A nature measuring apparatus (6) of to-be-processed water is provided at outer side and inside of cylindrical inner tank.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of microorganism reaction tank.    Microorganisms reaction tank (1)    Inner tank (3)    Upper portion of inner tank (3c)    Lower portion of inner tank (3d)    Control apparatus (4)    Control board (5)    Nature measuring apparatus (6)    Supply port (10)    Sedimentation fixed prevention apparatus (12)