A laser apparatus may include an optical resonator, a laser chamber, an optical loss adjustment mechanism, and a spectral line width adjustment mechanism. The optical resonator includes a mirror configured to reflect a part of light and a grating. The laser chamber is provided in the optical resonator and contains a laser gain medium, configured to emit a laser beam. The optical loss adjustment mechanism is provided in the optical resonator and configured to adjust an optical loss of the laser beam. The spectral line width adjustment mechanism is provided in the optical resonator and configured to adjust a spectral line width of the laser beam.