Abstract: The electrodeless discharge lamp of the present invention comprises: a bulb 1 provided with a substantially-spherical spherical portion 1a and a neck portion 1b extending from the spherical portion; a base 15 connected to the neck portion; a protrusion 4 formed at an apex of the spherical portion; and an induction coil 11a that causes light emission by discharge developed in the bulb. When defining the relations B=W/(4×p×(D/20)2), S=p×(d/20)2, L=p×(d/10), X=(B×S)/(L×A), where 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, then the electrodeless discharge lamp satisfies the formula below: t?6=10959×X+25=t+6 ??(Formula) where t is the temperature (° C.) at the tip of the protrusion 4 during downward stable lighting of the electrodeless discharge lamp.