A three-phase fluidized bed bioreactor is expected to treat wastewater quite efficiently, because the reactor has large specific surface area of biofilm, and oxygen for the biodegradation is simultaneously supplied from gas to liquid with oxygen consumption for biooxidation. However the reactor design and operation had been quite difficult particularly with a plug flow type fluidized bed, because of complicated behavior in the reactor. This paper concerns the current research work by the authors in focusing on the following characteristics and basic design method: (1) Behavior of biofilm immobilization on particulate media, fluidization of immobilized biofilm media and characteristics of oxygen absorption and substrates biodegradation. (2) Characteristics of biological treatment by immobilized biofilm. (3) Basic design method of a plug flow type three-phase fluidized bed bioreactor.