Abstract: A low-pressure mercury vapor-filled discharge lamp has a glass arc tube and a glass outer tube disposed coaxially with the arc tube forming a space therebetween. A gas is disposed in the space. The arc tube contains a gas and is coated with a phosphor. A first seal hermetically seals the inner tube. A second seal seals the inner tube to the outer tube. The inner tube further contains a pair of cathodes coupled to Dumet wires extending from the interior of the inner tube to the outside of the lamp structure. The pressure in the space is set at not more than 1 Pa, which is nearly high vacuum. The longer the radial dimension of the space, the greater the heat retaining capacity and the better the temperature characteristics which can be obtained. However, by setting the pressure of the space at 1 Pa or less, the optimum heat retaining capacity can be obtained while reducing the diameter of the low-pressure mercury vapor filled discharge lamp 1.