In an electrodeless discharge lamp 1 comprising a bulb 10 into which discharge gas and mercury which is controlled at a temperature of a coldest spot are filled, a power coupler 20 generating high frequency electromagnetic field, and a ferrule 15 for coupling the bulb 10 and the power coupler 20, the bulb 10 is configured of a substantially spherical barrel 14 formed of a transparent material and having an opening, and a sealing member 11 welded to the opening of the barrel 14 and having a substantially cylindrical cavity 5; a protrusion 4, which becomes a coldest spot when the lamp is lit in a state that the ferrule 15 is disposed upward, is formed at an apex of the bulb 10; and a protruding portion 17 is formed in a vicinity of just above the ferrule 15 of the bulb 10, that is, in a bulb neck portion 19 so that the bulb neck portion 19 becomes the coldest spot when the lamp is lit in a state that the ferrule 15 is disposed downward. Thereby, a constant optical output is obtained regardless of a posture of installation of the electrodeless discharge lamp 1.