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 VIN with respect to the pair of internal electrodes and an electric potential VH of the electrode of the higher electric potential of the pair of external electrodes to a condition where VIN>VH or VIN≈VH.