Abstract: Provided is a UV reaction apparatus for treating waste liquid caused by system decontamination in a nuclear power plant. The UV reaction apparatus comprises a UV irradiation unit for irradiating UV light as a bar-shaped body; and a transit unit accommodating the UV irradiation unit therein, and for passing organic waste liquid to be irradiated with the UV light, wherein the transit unit comprises, an input unit, into which the organic waste liquid is input a discharge unit, through which the organic waste liquid is discharged, and an intermediate unit located between the input unit and the discharge unit, and in which the UV irradiation unit is located.