Abstract: Provided are an apparatus and a method for biological treatment of an organic wastewater where it is possible to efficiently remove organic matter under high load while reducing the amount of air used for aerobic biological treatment and substantially decreasing the amount of sludge generated. The apparatus is provided with: a first biological treatment tank which has a fixed-type immobilized biocarrier and an aeration means; a second biological treatment tank to which a treated liquid from the first tank is introduced so as to perform treatment using suspended microorganisms, and which has an aeration means; and a sedimentation tank in which solid-liquid from the second tank is separated said liquid into settled sludge and treated water, wherein the apparatus has an influent line through which an organic wastewater is introduced to the first tank and the second tank, and a return line through which part of settled sludge discharged from the tank is returned to the tank.