An organic wastewater treatment device includes a biological treatment tank having a plurality of biological treatment units connected in series, where each biological treatment unit includes a pair of an anoxic tank disposed on an upstream side and an aerobic tank disposed on a downstream side along a flow of the organic wastewater, where a membrane separation device is immersed in activated sludge in the aerobic tank, a sludge return path from the aerobic tank on the most downstream side to the anoxic tank on the most upstream side, and an anaerobic tank for anaerobically treating the organic wastewater, which is then divided and supplied to the anoxic tank of each biological treatment unit. By repeating the denitrification in the anoxic tank and the nitrification in the aerobic tank. The membrane-permeated liquid from the membrane separation device in each biological treatment unit is discharged as treated water.