An electrodeless discharge lamp has a bulb into which a discharge gas and a mercury, which is controlled at a temperature of a coldest spot are filled. A power coupler generates a high frequency electromagnetic field. A ferrule couples the bulb and the power coupler. The bulb is configured as a substantially spherical barrel formed of a transparent material and has an opening. A sealing member is welded to the opening of the barrel having a substantially cylindrical cavity. A protrusion, which becomes the coldest spot when the lamp is lit in a state that the ferrule is disposed upward, is formed at an apex of the bulb. A protruding portion is formed in a vicinity just above the ferrule of the bulb, that is, in a bulb neck portion, so that the bulb neck portion becomes the coldest spot when the lamp is lit in a state that the ferrule is disposed downward. Thereby, a constant optical output is obtained, regardless of a posture of installation of the electrodeless discharge lamp.