<p>A pair of electrodes 3 are provided in an arc tube 1. I/S is 20 (A/mm<2>) or less on a condition that an area of a cross section at a tip of the electrodes 3 is S (mm <2>) and an operating current between the electrodes 3 is I (A). According to this constitution, a long lamp life can be obtained without causing a rise in the lamp voltage and the blackening of the arc tube 1 by the evaporation of the electrodes while operation time becomes longer. <IMAGE></p>
