PURPOSE: To suppress the formation of a harmful excess and deficiency reaction substance and the inorg. reaction of a modifying substance and to enhance the absorption and effective utilization efficiency of ozone by stepwise treating a contaminant using a waste ozone contact device, a recovered ozone contact device and a generated ozone contact device.

CONSTITUTION: A sludge component B is removed from raw water A in a pretreatment device 1 to obtain treated water C which is, in turn, introduced into a waste ozone contact device 2. This treated water C is supplied to a falling water stream D to be allowed to fall through a falling pipe 3 together with waste ozone X injected while sucked by compressed air T and reversed to a rising pipe 4 to form a rising water stream E which is, in turn, supplied to a head tank 5. The treated water is allowed to overflow a collecting return device 6 and a part thereof is returned to the falling pipe 3 and the remainder thereof is extracted to a separating return device 12. Biologically treated water F from which a fluidized biological carrier and peeled bacterial cells are removed by the separating return device 12 and a filter device 13 is introduced into a recovered ozone contact device 14. Further, iron/manganese water L mixed with the biologically treated water F which a fluidized introduced into a generated ozone contact device 26 through an iron removing device 25.