To improve the starting property and light flux build-up at a low cost by forming a light source with rare gas discharge lamps sealed with only the rare gas and fluorescent lamps sealed with mercury, and selectively lighting them according to the atmospheric temperature.

Rare gas discharge lamps 14 and mercury-containing fluorescent lamps 15 are arranged in parallel vertically at both ends of a light guide body 13 made of an acrylic resin rectangular plate. The atmospheric temperature is detected by a temperature sensor, only the discharge lamp 14 having a good starting property is lighted when the atmospheric temperature is the prescribed temperature or below, and the discharge lamp 14 is turned off and the fluorescent lamp 15 is lighted when the atmospheric temperature is the prescribed temperature or above. The light of the lamps 14, 15 is directly fed or reflected on a curved inner reflecting surface 12c and fed to the light guide body 13 from both ends, it is reflected on a reflecting film 17, then it is guided to a light outgoing surface 13a. Since the lamps 14, 15 are switched according to the atmospheric temperature, the starting property at a low temperature is good, high luminance is obtained on the light outgoing surface 13a in a wide temperature range. The lamps 14, 15 are arranged on the side of both ends of the light guide body 13, and a back-light 11 can be made thin.