PURPOSE: To prevent wear of machines and breakage of particles grown up by deposition of microoranisms by generating jet stream in a circulation passage to cause circulation in the circulation passage and stripping off deposited microorganisms from the grown-up particles in the passage.

CONSTITUTION: Waste water affords BOD component and N component contained therein to particles deposited with microorganisms forming fluidized bed while it flows upward through the fluidized zone 3, in a treating tank 1. Since the particles frown up by the deposition of microorganisms are floated up to the upper part near the boundary face of the fluidized bed zone 3 and solid/liqiud separation zone 5, they are entangled in the circulating stream formed in the circulation passage 11, 12, and a part of the microorganisms is stripped off. The circulating stream can be formed by ejecting the liquid in the upper part of the solid/liquid separation zone contg. substantially no solid from a nozzle provided to the end of a line 17 into a circulation passage through a line 13, pump 15 and line 17.