In an air-tight container made of transparent material, a phosphor, a gas or gas mixture and a pair of discharging electrodes are contained. The discharge gap is in the range of 0.1 to 3.0mm, and the pressure of the gas or gas mixture sealed in the container is such that the product of the pressure and the discharge gap is in the range of 30 to 300 Torr.mm. The gas or gas mixture has discharge radiation spectra within the region of wavelength shorter than 200nm. The phosphor is a europium activated yttrium gadolinium borate phosphor represented by the formula{ [ ( Y.sub.1-x, Gd.sub.x ).sub.1-z B.sub.z ].sub.1-y Eu.sub.y }.sub.2 O.sub.3where x, y and z are defined by the ranges of 0.ltoreq.x.ltoreq.1, 0.001.ltoreq.y.ltoreq.0.1 and 0.25.ltoreq.z.ltoreq.0.75. More preferably, x, y and z are defined by the ranges of 0.10.ltoreq.x.ltoreq.0.60, 0.005.ltoreq.y.ltoreq.0.05 and 0.50.ltoreq.z.ltoreq.0.65.