An EUV light source apparatus can reliably detect and accurately judge deterioration of an optical element in a laser beam focusing optics disposed within an EUV light generation chamber. This EUV light source apparatus includes: the EUV light generation chamber; a target material supply unit; an EUV light collector mirror; a driver laser; a window; a parabolic mirror which focuses collimated laser beam by reflection and is disposed within the EUV light generation chamber; an energy detector detecting energy of the laser beam diffused without being applied to a target material after being focused by the laser beam focusing optics when the EUV light is not generated; and a processing unit for judging the deterioration of the window and the parabolic mirror according to the laser beam energy detected by the energy detector.