An organic wastewater treatment method for biologically treating nitrogen-containing organic wastewater in activated sludge using an organic wastewater treatment device provided with (a) a biological treatment tank in which a plurality of biological treatment units are connected in series, each biological treatment unit having a pair of an anoxic tank disposed on an upstream side along a flow of the organic wastewater, and an aerobic tank disposed on a downstream side of the flow, a membrane separation device being immersed in activated sludge in the aerobic tank, and (b) a sludge return path for returning the activated sludge from the aerobic tank disposed on a most downstream side to the anoxic tank disposed on a most upstream side, the method comprising: dividing and supplying the organic wastewater to the anoxic tank of each biological treatment unit; biologically treating the organic wastewater by repeating a denitrification process in the anoxic tank and a nitrification process in the aerobic tank; discharging membrane-permeated liquid from the membrane separation devices of each biological treatment unit as treated water; and determining for each biological treatment unit whether to stop the membrane separation device which is operating, or whether to start the membrane separation device which has been stopped, based on at least one of an inflow amount of the organic wastewater, a tank water level, a transmembrane pressure difference of each membrane separation device, a T-N concentration of the treated water, and an NH 3 -N concentration of the treated water as an index, wherein a first partition wall is formed in each biological treatment unit between the anoxic tank and the aerobic tank, and a cutout is provided on a part of an upper end side of the first partition wall, such that the organic wastewater containing the activated sludge from the anoxic tank overflows into the aerobic tank, wherein a second partition wall is formed between each of the aerobic tank and each of the adjacent anoxic tank disposed on the downstream side along the flow of the organic wastewater, and an outflow port for the organic wastewater containing the activated sludge is provided at a position of the second partition wall corresponding to the vicinity of the bottom of the membrane separation device in the vertical direction, such that the organic wastewater containing the activated sludge from the aerobic tank flows into the anoxic tank, wherein an upper end of an opening which serves as the outflow port is submerged, and is disposed at or lower than 30 cm from a water surface of the aerobic tank, wherein an outflow rate of the activated sludge from the outflow port is set equal to or smaller than 0.5 m/sec, and wherein an air lift pump is provided at the anoxic tank on the most upstream side, air bubbles are supplied from a blower to an air lift pipe through a valve so as to generate an upward flow in the air lift pipe, by which the organic wastewater containing the activated sludge from the anoxic tank on the most upstream side is sent to the aerobic tank on the most upstream side by the pump. dividing and supplying the organic wastewater to the anoxic tank of each biological treatment unit; biologically treating the organic wastewater by repeating a denitrification process in the anoxic tank and a nitrification process in the aerobic tank; discharging membrane-permeated liquid from the membrane separation devices of each biological treatment unit as treated water; and determining for each biological treatment unit whether to stop the membrane separation device which is operating, or whether to start the membrane separation device which has been stopped, based on at least one of an inflow amount of the organic wastewater, a tank water level, a transmembrane pressure difference of each membrane separation device, a T-N concentration of the treated water, and an NH 3 -N concentration of the treated water as an index, wherein a first partition wall is formed in each biological treatment unit between the anoxic tank and the aerobic tank, and a cutout is provided on a part of an upper end side of the first partition wall, such that the organic wastewater containing the activated sludge from the anoxic tank overflows into the aerobic tank, wherein a second partition wall is formed between each of the aerobic tank and each of the adjacent anoxic tank disposed on the downstream side along the flow of the organic wastewater, and an outflow port for the organic wastewater containing the activated sludge is provided at a position of the second partition wall corresponding to the vicinity of the bottom of the membrane separation device in the vertical direction, such that the organic wastewater containing the activated sludge from the aerobic tank flows into the anoxic tank, wherein an upper end of an opening which serves as the outflow port is submerged, and is disposed at or lower than 30 cm from a water surface of the aerobic tank, wherein an outflow rate of the activated sludge from the outflow port is set equal to or smaller than 0.5 m/sec, and wherein an air lift pump is provided at the anoxic tank on the most upstream side, air bubbles are supplied from a blower to an air lift pipe through a valve so as to generate an upward flow in the air lift pipe, by which the organic wastewater containing the activated sludge from the anoxic tank on the most upstream side is sent to the aerobic tank on the most upstream side by the pump.