A driver laser for EUV light source apparatus which driver laser simultaneously achieves short-pulsing and multi-line oscillation. The driver laser includes: a short-pulse multi-line oscillated CO.sub.2 laser oscillator having a device that shortens width of pulses included in a laser beam to be output and a device that suppresses amplitude of an oscillation spectrum exhibiting an energy peak value; and at least one amplifier that inputs the laser beam output from the short-pulse multi-line oscillated CO.sub.2 laser oscillator and amplifies the input laser beam to output the amplified laser beam.