PURPOSE: To surely prevent the propagation of bacteria and to improve the operating efficiency of a recovering device by previously filling the inside of an ion exchange device with a regenerating agent during the period when the water passage is halted in the method for preventing the propagation of the bacteria during the halt of the water passage.

CONSTITUTION: The waste water recovering device is representatively constituted by having an active carbon filter 1 and the ion exchange device 2 connected to the downstream side thereof and by using, for example, a UV oxidizing device 3 and an active carbon filter 4 of a rear stage at need. The ion exchange device 2 is internally filled with the regenerating chemical of about 0.5 to 10wt.% during the operation halt of the waste water recovering device, i.e., the period when the water passage is halted. Namely, the regenerating chemical of the prescribed concn. is admitted into the ion exchange device 2 by utilizing the regenerating device to substitute the water in the ion exchange device 2 with the regenerating chemical; thereafter, the ion exchange device 2 is halted in the state of filling the device with the regenerating chemical, by which the propagation of the bacteria is prevented during the halt of the water passage.