The present invention provides an apparatus for producing photocatalytic reaction water through a photocatalytic reaction, which can produce water containing a satisfactory amount of active oxygen species, can eliminate microorganisms, parasites or protozoa, shows high oxidizing ability for a prolonged period of time, can reduce the power requirements, is small in size, and is applicable to various devices. A photocatalyst is radiated with light emitted from a light source to produce active oxygen species, and the active oxygen species is diffused in water, whereby the water is provided with functions of the active oxygen species. An oxidation reaction with the water is utilized to perform at least one selected from the elimination of microorganisms, the elimination of parasites, and the elimination of protozoa.