The control of the spectral purity width E95 is performed while imparting practically no effect to the control of a central wavelength, and the spectral purity width E95 is stabilized. A wavefront adjuster 32 is provided on an output side of the interior of an optical resonator, i.e., on an output coupler 31 side. Light generated in a laser chamber 10 is transmitted through the wavefront adjuster 32 from the laser chamber 10 side, and reaches the output coupler 31. In the wavefront adjuster 32, the distance between concave and convex lenses 33 and 34 is adjusted so that a desired spectral purity width E95 can be obtained. Then, when the light passes through the wavefront adjuster 32, the wavefront of the light is adjusted to a desired wavefront.