PURPOSE: To effectively utilize the whole volume of a reaction tank for treatment of waste water by utilizing magnetic force to concentrate activated sludge forcedly, and efficiently returning the concentrated activated sludge to the inlet side of the reaction tank.

CONSTITUTION: Powdery magnetic substance 16 is added to activated sludge liquid in a reaction tank 2. The activated sludge is transferred to a separation tank 1 placed in the outlet 2a side of the reaction tank 2. Thereafter a magnetic field is impressed in the separation tank 1 by a magnetic generator 10. Thereby activated sludge 14 holding the powdery magnetic substance 16 is caught on the magnetic generator 10 and forcedly flocculated. Since activated sludge 14 flocculated in the separation tank 1 still has sufficient fluidity in the treated liquid, impression of the magnetic field is stopped for a time. In the meanwhile, the concentrated activated sludge liquid containing flocculated activated sludge 14 is returned to the inlet 2a side of the reaction tank 2 through a pipeline 13.