To radiate a large quantity of ultraviolet rays without reducing the ultraviolet rays in a short wavelength band, and prevent the generation of a stain in an inner wall of a tube by changing the shape of a glass tube so as to control the vapor pressure of mercury.

Inner diameter of a glass tube 2 is contracted so that a discharge tip 3a of a cathode discharge electrode 3 to be sealed in the glass tube 2 is projected from the end surface of the mercury of a mercury reservoir. The inner diameter of the tube 2 is contracted so that a discharge tip 4a of an anode discharge electrode 4 is arranged inside the mercury end surface 5a so as to form other mercury reservoir. Thickness of the tube 2 in the mercury reservoir, temperature of the cooling water 40 of the wall of the tube 2 and lamp input are constantly maintained, and evaporation area of the mercury end surface 5a is adjusted so as to control the mercury vapor pressure. With this structure, plasma positive column having the same diameter with the inner diameter of the tube 2 is formed, and the absorption of the ultraviolet rays in the short wavelength band in a gap space, in which mercury atoms between the electrodes 3, 4 exist, is prevented, and the generation of a stain near the tube wall is settled in one direction.