An aim of the invention is to provide a bioreactor and a waste water treatment method that employs this tank, whereby anaerobic and aerobic microorganism treatment can be continuously carried out, even when the volume of the bioreactor is large, and whereby installation costs can be minimized. The invention comprises an outer tank (2), a cylindrical inner tank (3) disposed inside this outer tank and having openings above and below, a circulation rate control device (4) for controlling the circulation rate of water to be treated in the bioreactor, the circulation rate control device (4) being provided at an upper part of this cylindrical inner tank, a cylindrical control plate (5) for bringing about sedimentation of sludge, the cylindrical control plate (5) being provided at the outer circumference of an upper part of the cylindrical inner tank, a treated-water quality measurement device (6) that is provided outside and inside of the cylindrical inner tank, a waste water supply opening (10) that is provided in
