A light source device that irradiates a discharge vessel with a laser beam to produce radiant light that is reflected by an ellipsoidal reflecting surface efficiently utilizes the light produced by directing the laser beam through an unirradiated region where reflected light from the ellipsoidal reflector is blocked by the discharge vessel, through an opening side of the ellipsoidal reflector to the discharge vessel. The discharge vessel has an emission substance enclosed inside which is excited by the laser beam and produces radiant light, is arranged at a focal point of the ellipsoidal reflector. A planar mirror, with which radiant light reflected by the ellipsoidal reflector is reflected in a different direction has a window in an unirradiated region where reflected light from the ellipsoidal reflector is blocked by the discharge vessel through which the laser beam passes to the discharge vessel.