PURPOSE: To provide a stable and long-lived light source for exposure enabling formation of a phosphor layer giving high illuminance and high efficiency, by making the inside diameter of the discharge portion of the bulb smaller than that of the portion where the electrode and the mercury pool are provided and by equipping heater means and cooler means at the external terminal which is fixed to the electrode.

CONSTITUTION: Each of the tungsten electrodes 32 to be sealed in the end portions of the bulb 31 of qualtz glass to oppose each other is attached by welding etc. to a thin metal plate 33 having the thermal expansion coefficient conforming to that of the qualtz glass, and the metal plate is sealed in the end portion of the bulb at a predetermined position. Mercury 34 and small amounts of xenon gas etc. are sealed in the portion of the bulb where the electrode is provided. The inside diameter of the discharge portion 35 is made smaller than that of the portion 36 where the electrode is provided. The external terminal 37 is pressed in and fixed to the metal plate. The external terminal is equipped with both heater means 38 and cooler means 39.