The purpose of the present invention is to obviate the need for various equipment as a measure against damage to a protective tube (quartz tube) in which a UV lamp is inserted, while having water pressure resistance equivalent to that of an internal-irradiation-type configuration. The present invention is provided with a UV-transmitting protective tube (2) accommodated in a pressure-resistant container (1), a UV lamp (3) accommodated in the protective tube, and UV-transmitting water passage tubes (4) accommodated in the container (1). A liquid (6) to be processed flows in the water passage tubes. Residual space in the container (1) is filled with a UV-transmitting liquid medium (5), and ultraviolet rays from the UV lamp (3) pass through the protective tube (2), the liquid medium (5), and the water passage tubes (4), and are radiated to the liquid (6) to be processed. The water passage tubes (4) are disposed in the liquid medium, and the pressure is thereby substantially equal inside and outside the water passage tubes (4), the water pressure resistance of the water passage tubes is substantially equal to the water pressure resistance of the container, and there is no need to specially reinforce the water passage tubes. The liquid (6) to be processed in the water passage tubes also does not contact the protective tube (2), and there is therefore no need for various equipment as a measure against damage to the protective tube.
