A method of controlling a laser apparatus may include: exchanging a gain medium in a chamber configured to output a laser beam by exciting the gain medium; first measuring, after the exchanging, pulse energy of a laser beam which is oscillated in the chamber under a specific gas pressure and a specific charge voltage; calculating an approximate expression indicating a relationship between the pulse energy of the laser beam and the gas pressure in the chamber and the charge voltage, or a table representing a correlationship between the pulse energy, the gas pressure and the charge voltage, based on the specific pressure, the specific charge voltage and the pulse energy in the first measuring; storing the approximate expression or the table; second measuring, after the first measuring, pulse energy Er of a laser beam oscillated in the chamber; calculating pulse energy Eec which is supposed to be obtained directly after the exchanging under the gas pressure and the charge voltage in the second measuring based on the approximate expression or the table; calculating a reduction amount ΔEd of pulse energy based on the pulse energy Eec and the pulse energy Er using ΔEd=Eec−Er; and calculating a partial gas exchange amount Q for partial gas exchange in the chamber based on the reduction amount ΔEd of pulse energy.