NOVELTY - The submerged membrane bio-reactor comprises a submerged membrane module (10) having a membrane with a hollow fiber structure, a cylindrical tube (20) covering an outer circumference of the submerged membrane module, a slender tube shaped nozzle (30) provided at a lower side of the cylindrical tube, and a porous diffuser (40). The nozzle and the diffuser are provided within a treatment vessel (50) to supply air to inside of the cylindrical tube. The nozzle has a diameter of 0.1-10 mm. A cross-sectional area ratio (Am/At) of the membrane to the cylindrical tube is 0.25-0.30.    USE - The submerged membrane bio-reactor is useful for sewage and wastewater treatment.    ADVANTAGE - The submerged membrane bio-reactor is devoid of contamination, and is effective and reliable for sewage and wastewater treatment thus obtaining a stable amount of reusable water.    DETAILED DESCRIPTION - The submerged membrane bio-reactor comprises a submerged membrane module (10) having a membrane with a hollow fiber structure, a cylindrical tube (20) covering an outer circumference of the submerged membrane module, a slender tube shaped nozzle (30) provided at a lower side of the cylindrical tube, and a porous diffuser (40). The nozzle and the diffuser are provided within a treatment vessel (50) to supply air to inside of the cylindrical tube. The nozzle has a diameter of 0.1-10 mm. A cross-sectional area ratio (Am/At) of the membrane to the cylindrical tube is 0.25-0.30, and the nozzle having a cross-sectional area ratio (Am/At) of the membrane to the cylindrical tube is 0.50-0.60, where Am is a cross-sectional area of the membrane in the submerged membrane module and At is a cross-sectional area of the cylindrical tube. The cylindrical tube has a cylindrical body and a conical lower portion formed on the cylindrical body, where the lower portion has a gradually increasing inside diameter larger than that of the cylindrical body.    DESCRIPTION OF DRAWING(S) - The diagram shows a submerged membrane bio-reactor.    Submerged membrane module (10)    Cylindrical tube (20)    Nozzle (30)    Porous diffuser (40)    Treatment vessel. (50)