PURPOSE: To irradiate not only the part near an outflow port but the inside of a valve as well with UV rays so that contamination can be prevented by providing an outflow opening/closing means consisting of a UV ray transmittable material in the bottom of a tank in such a manner that the means is irradiated by the UV rays with which the inside of the tank is irradiated.

CONSTITUTION: The water to be treated is irradiated with the UV rays from a sterilizing lamp 5 and is thereby made into sterilized water which is taken out through an outflow conduit 8 to the outside of the device. A lower part 10 and outflow port 11 of the conduit 8 exposed to the atmosphere are irradiated with the UV rays transmitted through a bottom plate 7 or the conduit 8 from the sterilizing lamp 5 over the entire part and, therefore, the contamination of the outflow port is prevented. Since the outflow opening/closing means 15 consists of the UV ray transmittable material and is simultaneously irradiated with the UV rays transmitted through the bottom plate 7 or the conduit 8 from the sterilizing lamp 5 fully down to the inside, the means is sterilized. The contamination of the outflow opening/closing means 15 itself and the inside thereof is thus obviated.