To control quality of laser beams of a spreading angle of the laser beams, etc., by a method wherein a voltage waveform or a current waveform between main electrodes is measured, and a state of a laser medium gas in a gas sealing container is changed, based on a result obtained by comparing the above with a reference value in the voltage waveform or the current waveform.

A voltage in an initial state is measured, and next a voltage during oscillation is measured. A comparison is made whether or not a difference between both the voltages is greater than a predetermined value. If greater, a halogen gas is injected, and if smaller, a voltage during the oscillation is continued to measure. The injection of the halogen gas is later repeated while measuring a difference between both the voltages, and it is confirmed whether or not the number of times of repetition becomes thrice. If within three times, further continue, and if exceeding three times, a krypton gas is injected. The injection of the krypton gas is later repeated while measuring a difference between both the voltages, and it is confirmed whether or not the number of times of repetition becomes thrice, and if exceeding thrice, a demand for gas countermeasure is issued.