The wastewater treatment system according to the present invention includes a plurality of reaction sequences including a first sedimentation tank, a reaction tank, a final sedimentation tank, a first channel which connects the first sedimentation tank and the reaction tank and a second channel which connects the reaction tank and the final sedimentation tank. In one reaction sequence of the plurality of reaction sequences, the reaction tank has a membrane separation tank including a carrier, a membrane unit, and activated sludge and in which an MLSS concentration of the activated sludge is in a range of 500 mg/L to 7000 mg/L, and wastewater is supplied to the reaction tank via the first channel, and wastewater in a quantity exceeding a treatment capacity of the reaction tank is supplied to the final sedimentation tank via the second channel.