A light source of the invention includes: a cylinder having disposed therein a phosphor material that emits light by ultraviolet rays which are radiated due to discharge; a pair of internal electrodes disposed inside the cylinder; a pair of external electrodes a and b disposed outside the cylinder; and a lamp controller that switches between an external electrode lighting mode resulting from the application of a voltage to the pair of external electrodes and an internal electrode lighting mode resulting from the application of a voltage to the pair of internal electrodes, wherein the lamp controller controls, in the external electrode lighting mode, an electric potential V_IN with respect to the pair of internal electrodes and an electric potential V_H of the electrode of the higher electric potential of the pair of external electrodes to a condition where V_INV_H or V_INV_H.