PURPOSE: To perform the sterilization of tap water with good efficiency, by providing an ozonizer, an ozone meter, an activated carbon tower arranged on the way of a piping route, compressor for sending air into a tank and an activated carbon tower for decomposing residual ozone.

CONSTITUTION: Piping for supplying ozone to a tank 1 from the ozonizer 6 provided to the outside of the tank 1 through a valve 7 is provided and ozone is passed through the stored water 4 in the tank 1 from the air diffusion pipe 8 attached to the terminal of the piping in said tank 1 perform sterilization. The residual ozone in stored water 4 is detected by an ozone meter 13 and, when no residual ozone is detected, the ozonizer 6 is again operated to supply ozone into stored water. Unreacted ozone is introduced into an activated carbon tower 9 from the tank 1 through a valve 10 and discharged to the open air through a valve 11 while receiving decomposition treatment.