The object is to provide an activated sludge treatment method whereby production of excess sludge is brought substantially to zero; and a method for upgrading existing waste water treatment equipment, employing this method. The method includes a step (1) in which sludge (2) produced in a sedimentation tank is supplied to a waste water aeration-conditioning tank for activated sludge treatment; a step (2) in which waste water into which the sludge (2) has been mixed is subjected to aeration-conditioning to bring the oxidation-reduction potential to a positive value, and supplied to a pressurized flotation concentration-separation tank; a step (3) in which the sludge (1) produced through treatment in the pressurized flotation concentration-separation tank is supplied to the bioreactor; a step (4) in which the treated waste water (1) separated from the pressurized flotation concentration-separation tank is supplied to a dilute activated sludge aeration tank; a step (5) in which released water and the sludge (3) produced by treatment in the bioreactor are supplied to the dilute activated sludge aeration tank; and a step (6) in which the treated waste water (2) separated from the dilute activated sludge aeration tank is supplied to the sedimentation tank.