A part of pre-treated water flowing into an ultraviolet irradiation tank is guided to a fluorometer. The fluorometer scans an excitation spectrum peak wavelength of the pre-treated water at a fluorescence wavelength fixed to 425 nm to obtain an excitation spectrum, and continuously measures an excitation peak wavelength thereof. Based on the analysis result obtained by the fluorometer, an ultraviolet irradiation device calculates an ultraviolet irradiation level target value for optimizing an ultraviolet irradiation level, and thus controls the irradiation level of ultraviolet rays emitted therefrom.