To efficiently recover treated water of high quality by stabilizing and enhancing efficiency of advanced treatment by reducing an amount of organic matter flowing into a latter stage of the advanced treatment process by reducing a bio-metabolic amount formed in a biological treatment process, in the treatment of organic matter-containing waste water by performing the biological treatment in a preceding stage of the advanced treatment like membrane separation.

Anaerobic biologically treated water in an anaerobic reactor tank 11 comprising an acid forming tank 11A and a UASB reactor tank 11B is coagulated and separated into solid and liquid in a settling tank No.1, (14). The obtained separated water is biologically and aerobically treated in an aerator 21, aerobic biologically treated water is coagulated, and separated into solid and liquid in a settling tank No.2, (24). Then, obtained liquid is treated in an RO membrane separation device 40 as an advanced treating means. By the anaerobic biological treatment and aerobic biological treatment in the preceding stage of the advanced treatment means, and solid/liquid separation, organic matter is adequately removed, to make the treatment by the advanced treatment means a stable and efficient one.