The electrodeless discharge lamp comprises: a bulb provided with a substantially-spherical spherical portion and a neck portion extending from the spherical portion; a base connected to the neck portion; a protrusion formed at an apex of the spherical portion; and an induction coil that causes light emission by discharge developed in the bulb. The electrodeless discharge lamp satisfies the formula below: t−6□10959×X+25□t+6  (Formula) where X=(B×S)/(L×A), B=W/(4×π×(D/20)2), S=π×(d/20)2, L=π×(d/10), W (W) denotes the lamp input power, D (mm) denotes the diameter of the spherical portion, d (mm) denotes the diameter of a portion at a joint surface between the neck portion and the base, and A (mm) denotes the distance from a largest-diameter portion of the spherical portion to the joint surface, and t is the temperature (° C.) at the tip of the protrusion during downward stable lighting of the electrodeless discharge lamp.