Luminous flux and luminous efficiency of a 40 watt fluorescent lamp comprising a ballast specified by the I.E.C. Publication 82 (covering Ballast for fluorescent lamps) is greatly improved by employing an improved tube characterized by: HAVING THE NOMINAL TUBE LENGTH OF 1,200 MM AND THE INNER DIAMETER OF 20 MM TO 25 MM, PROVIDED INSIDE AT EACH END OF THE TUBE WITH AN ELECTRODE E having the length l of 15 mm to 55 mm inward (, from the end face of each cap C as measured as shown in FIG. 2,) and containing mixed rare gas inside the tube, the rare gas having composition ratio represented by any point within a diagonal region defined by connecting the following points (a) to (e) and (a) with straight lines in the following order, on a trilinear chart for xenon, krypton and argon mixture: A.Xe: 1%, Kr: 0%, A: 99% b. Xe: 15%, Kr: 0%, A: 85% c.Xe: 15%, Kr: 65%, A: 20% d.Xe: 0%, Kr: 80%, A: 20% e.Xe: 0%, Kr: 5% A: 95% the pressure of the mixed gas being between 1.0 mmHg and 3.5 mmHg at the temperature of 20*C.