A charging voltage Vosc applied to a main capacitor C0 disposed in an oscillating high-voltage pulse generator 12 of an oscillating laser 100 is subject to constant control such that a pulse energy Posc of the oscillating laser 100 becomes a lower limit energy Es0 or more of an amplification saturation region. And, a charging voltage Vamp applied to a main capacitor C0 disposed in an amplifying high-voltage pulse generator 32 of an amplifying laser 300 is controlled, and pulse energy Pamp of the amplifying laser 300 is determined as target energy Patgt. Thus, the pulse energy of a two-stage laser is controlled to stabilize the pulse energy.