To provide a light source device to improve utilization efficiency of electroluminescent light.

The light source device is equipped with: a light source 1 to emit light by discharge of xenon gas enclosed inside an arc tube 4; and a water-cooled tube 2 arranged by keeping a predetermined gap surrounding the arc tube 4, and is constituted such that cooling water is introduced into the gap and the cooling water introduced is drained. On an outer perimeter 4a of the arc tube 4, on an inner perimeter 2b of the water-cooled tube 2, or an outer perimeter 2a of the same, fluorescent paint 5 which emits light having a wavelength included in a required wavelength band in the electroluminescent light emitted by the light source 1, excited by irradiation of light having a shorter wavelength than the light of required wavelength, is applied in such a suitable thickness that the light with the required wavelength emitted by electroluminescence from the light source 1 is superimposed by the light emitted by the fluorescent paint 5 by excitation and to be emitted externally.