PURPOSE: To suppress the movement of the point where an arc develops during electric discharge by sealing an anode and a cathode facing the anode and having a pointed head, which is formed by impregnating a porous high-melting metallic base body with an easily-electron-discharging substance, in an atmosphere of electric discharge gas.

CONSTITUTION: After an anode 15 and a cathode 16 are installed facing each other in a quartz emission tube 11, the tube 11 is charged with xenon gas thereby making a light- source electric discharge tube. A cathode 16 is made by attaching an end section 21 to the end of a molybdenum or tungsten bar 20 having a diameter of D; the end section 21 has a length of L and made by impregnating a porous metallic base body, which has a porosity of 10W35% and is formed by subjecting a tungsten powder with a mean particle diameter of 2W8μ to pressure molding before the molded body is sintered in a vacuum, with an electron-discharging substance prepared from an alkaline earth metal aluminate. The diameter (D) and the length (L) are adjusted according to the relationship 0.1<L/D<6. By the means mentioned above, it is possible to effectually prevent any deformation or denaturation of the end section 21 which might be caused by electric discharge, thereby enabling a light source causing a small movement of the luminous point to be produced.