This invention relates to an excimer laser oscillation apparatus which has a laser chamber (20) which stores a laser gas containing a gas mixture of at least one inert gas selected from the group consisting of Kr, Ar, and Ne, and F 2 gas, and in which an inner surface thereof has a reflection-free surface (3) with respect to light of a desired wavelength of 248 nm, 193 nm, or 157 nm, and the uppermost surface of the inner surface consists of a fluoride, an optical resonator which is made up of a pair of reflection mirrors (5,6) arranged to sandwich the laser chamber (20) therebetween, and in which the reflectance of the reflection mirror (6) on the output side is 90% or more and microwave introduction means (300,303,302), arranged on the laser chamber (20), for continuously exciting the laser gas in the laser chamber (20).