The present invention provides an ultraviolet light irradiation device having a planer UV light source in which the irradiation intensity of UV light can be adjusted finely in a wider range. The ultraviolet light irradiation device of the present invention comprises an UV light source and a housing that holds the planer UV light source. In the UV light source, a plurality of thin plasma tubes, each of which has an UV phosphor layer formed therein, are arranged in parallel with each other on an electrode support sheet, and drive circuits apply a pulse voltage to electrode pairs provided between the electrode support sheet and an array of the thin plasma tubes. A control circuit controls a factor of the pulse voltage to be applied to the electrode pairs so as to adjust the irradiation intensity of the UV light.