A gas discharge device includes a thin glass tube filled with a discharge gas; a pair of first and second long electrodes extending toward either side along a longitudinal direction with a discharge gap interposed therebetween are provided outside of a back side flat surface of a thin glass tube; and a ultraviolet phosphor layer formed on an inner surface at the back side flat surface, the thin glass tube filled with a discharge gas having a front side flat surface and the back side flat surface facing each other on a transverse section, wherein, starting with trigger discharge that is initially generated in the discharge gap as a result of a voltage increase when a voltage with a sine waveform or an inclined waveform is applied between both electrodes, the discharge gradually extends so as to move in the longitudinal direction of the electrodes. Ultraviolet light having high luminous efficiency and emission intensity is obtained from a front side surface of the thin glass tube by driving the device with a sine-wave AC voltage.