The inventive device efficiently cools, with a simple construction, heat generated from an ultraviolet light-emitting diode (UV-LED). The UV-LED is accommodated in a housing that has an open end and an ultraviolet-transparent closing end. A portion of the housing adjacent to the closing end contacts to-be-treated liquid, ultraviolet rays generated from the UV-LED are irradiated to the to-be-treated liquid, and the housing is cooled by the to-be-treated liquid. In this way, the heat generated from the UV-LED can be cooled with a simple construction without use of arrangements for introducing dedicated cooling fluid for cooling the UV-LED. A heat discharge block for discharging to the outside the heat generated from the UV-LED may be provided at or adjacent to the open end of the housing. Further, the present invention may be constructed in such a manner that a portion of the discharge section directly contacts the to-be-treated liquid.