To prevent the formation of slime and scale to keep the filtration performance of a filter membrane for a long period of time by immersing and arranging a filter membrane module in organic sewage water in a biological treating vessel in which microoganism carriers are mixed in organic sewage water.

In a fluidized bed type biological treating device 1, an organic sewage water inflow pipe 6, a sludge withdrawal pipe 8 and a diffusing means 5 for jetting gas are arranged respectively in the upper part, in the lower part and in the bottom part thereof and microoganism carriers 4 are mixed in sewage water to biologically perform purifying treatment. A filter membrane module 2 is constituted by arranging plural pieces of hollow yarn or the like using a microfiltration membrane of about 0.1-1μm pore diameter and an ultrafiltration membrane having several tens of thousands to several thousands of thousands of fractional molecular weight, both of which are an organic membrane of polyethylene, cellulose acetate or the like. A discharge pipe 7 is connected to a permeation liquid flow passage for together discharging permeation liquid discharged from the filter membranes, and evacuation and filtration are performed by a withdrawal pump 11.