The amount of pre-ionization by the pre-ionization electrode provided on the gas outflow side of the main discharge space, out of at least one pair of pre-ionization electrodes, is adjusted so as to attain the desired beam profile. As a result, the pre-ionization intensity of the laser gas remaining on the gas outflow side of the main discharge space is made lower than the pre-ionization intensity of the laser gas flowing into the main discharge space. In other words, the intensity of the laser light on the gas outflow side of the main discharge space is made weaker than the intensity of the laser light in the center of the main discharge space. Accordingly, the location of the maximum light intensity of the beam profile does not deviate from the center of the discharge width. Also, since the pre-ionization electrodes are provided on both the gas inflow side and the gas outflow side of the main discharge space, the pre-ionization intensity of the laser gas on the gas inflow side of the main discharge space is never stronger than the pre-ionization intensity of the laser gas present at the center of the main discharge space.
