PURPOSE: To increase the temperature at the coolest position and to improve the luminous efficacy by reflecting infrared rays radiated from a luminous tube so as to illuminate the coolest position of the luminous tube.

CONSTITUTION: When a luminous tube 2 is applied with a voltage and is started, heat is generated in conjunction with an arc discharge, sealed sodium evaporates, the vapor pressure is increased, the lamp voltage is increased, and the lamp luminesces. About 1/3 of the radiated energy is visible rays, about 1/3 is radiated as infrared rays, and remaining about 1/3 is dissipated through an outer tube as the radiation energy. Visible rays are transmitted through an infrared ray reflector 10 as shown by an arrow A and are radiated outside as the light energy. And infrared rays are reflected by the infrared reflector 10 with a shape of a hyperbolic rotor as shown by an arrow B, and the coolest position at the lower end portion near an electrode is illuminated and heated with reflected infrared rays, the temperature is increased, thus accelerating the evaporation of sodium and increasing the vapor pressure, thereby the luminous efficacy is improved.