A film is formed on the discharge parts of the main discharge electrodes. In order to prevent erosion of the discharge parts by the halogen gas contained in the laser gas, a substance that tends not to react with the halogen gas, i.e., a halogen-resistant substance, is used for this film. Furthermore, in order to prevent deformation of the discharge parts by the bombardment and heat of the main discharge, a substance that has a higher hardness than the metal of the main discharge electrodes or a substance that has a higher melting point than the metal of the main discharge electrodes is used for this film. As a result, deterioration of the electrodes can be inhibited, so that a stable laser output can be obtained, and the replacement interval of the electrodes can be extended.