An excimer laser apparatus may include an optical resonator, a chamber including a pair of discharge electrodes, the chamber being provided in the optical resonator and configured to store laser gas, an electric power source configured to receive a trigger signal and apply a pulsed voltage to the pair of discharge electrodes based on the trigger signal, an energy monitor configured to measure pulse energy of a pulse laser beam outputted from the optical resonator, a unit for adjusting partial pressure of halogen gas configured to perform exhausting a part of the laser gas stored in the chamber and supplying laser gas to the chamber, and a controller configured to acquire measurement results of the pulse energy measured by the energy monitor, detect energy depression based on the measurement results of the pulse energy, and control the unit for adjusting partial pressure of halogen gas based on results of detecting the energy depression to adjust the partial pressure of halogen gas in the chamber.