A laser oscillator that can suppress astigmatism and achieve an axially symmetric laser beam output profile even when a spherical mirror is inserted between an output mirror and a rear mirror. Turning mirrors (plane mirrors) and spherical mirrors are arranged between the output mirror and the rear mirror so as to provide a zigzag round-trip optical path where the optical axes cross in the same plane. This configuration achieves the same effect as if lenses were arranged between the ends of a very long resonator, and the light-gathering performance of the laser beam may be controlled by the output mirror, the rear mirror and the curvatures of the respective turning mirrors. The positions and orientations of the spherical mirrors are determined so that the planes formed by the optical axis of the incident laser beam and the optical axis of the reflected laser beam on the respective spherical mirrors are perpendicular to each other, thereby offsetting the astigmatism and preventing the axial symmetry of the output laser beam profile from degrading. The angle of incidence of the laser beam axis on each spherical mirror may be set to 22.5 degrees or larger. Further, the turning mirrors may be replaced by a slab crystal.