A laser oscillator configured to limit a mode hopping over a long duration. A laser oscillator has an optical resonator including an output coupler and a rear mirror positioned on an optical axis, at least one folding mirror positioned on the optical axis and between the output coupler and the rear mirror, and a discharge tube positioned between the output coupler or the rear mirror and the folding mirror. At least one folding mirror has a toric surface shape, a saddle surface shape or a cylindrical shape, and is configured to rotate about a straight line as a rotation axis, which extends through one point on a surface of the folding mirror and is perpendicular to the surface of the folding mirror.