Disclosed is a bioreactor for treating sewage comprising an aerobic tank including a mixing cell for receiving sewage to be supplied from an inlet and mixing the sewage with activated sludge and an aerobic reactor tank in which the activated sludge adsorbs organic substance existing in the sewage; a backwashing cartridge filter for removing floc resulting from growth of the activated sludge adsorbing the organic substance; and an anaerobic tank for carrying out a denitrification process for denitrifying treated water flowed through the backwashing cartridge filter using anaerobic ammonium oxidation (anammox) bacteria wherein the backwashing cartridge filter allows the sewage discharged from the aerobic tank to pass through the cartridge filter and separates the floc and the treated water using difference in size between the floc contained in the sewage and pores in the cartridge filter, and wherein foreign matter adsorbed on the cartridge filter is easily removed by means of washing water to be injected into the cartridge filter.