The invention water tank, mixing tank, coagulation tank, a settling tank, a separator tank, and; But when supplied to the concentrated water containing the filtered sludge by immersion type separation membrane device in the separator tank comprises a concentration membrane filtration tank to the second membrane filtration drive discharged to be processed, of the concentrated water containing the filtered sludge in the separator tank, the sludge a high concentration layer comprising a high concentration is a low concentration layer comprising a sludge remaining in the return twos the aggregation for a set time is made the coagulation, sedimentation and membrane filtration, and then the set time has elapsed, after the high concentration layer is discharged to the relatively low concentrations It is supplied to the dense membrane filtration tank, and wherein the second made is primarily membrane filtration, whereby the concentrated membrane filtration tank is supplied only concentrate containing the sludge is relatively at a low concentration being a filtering be done, concentrated film, etc. are clogged by the sludge the bar of contamination decreases, the solid-liquid separation effect of the concentrated membrane filtration tank This increase is, it is possible to drive a long period of time without cleaning operation is ultimately to improve the purification efficiency.