An extreme ultraviolet light source apparatus using a spectrum purity filter capable of obtaining EUV light with high spectrum purity. The apparatus includes a chamber; a target supply unit for supplying a target material; a driver laser using a laser gas containing a carbon dioxide gas as a laser medium, for applying a laser beam to the target material to generate plasma; a collector mirror for collecting and outputting the extreme ultraviolet light radiated from the plasma; and a spectrum purity filter provided in an optical path of the extreme ultraviolet light, for transmitting the extreme ultraviolet light and reflecting the laser beam, the spectrum purity filter including a mesh having electrical conductivity and formed with an arrangement of apertures having a pitch not larger than a half of a shortest wavelength of the laser beam applied by the driver laser.