A laser oscillator includes: a plurality of laser media to emit laser beams having different wavelengths; a diffraction grating to emit, in a superimposed state, the laser beams incident from the laser media; a partially reflective element to reflect part of the laser beams emitted from the diffraction grating and return the part of the laser beams to the diffraction grating, and to transmit a remainder; and a plurality of lenses each disposed between a corresponding one of the laser media and the diffraction grating. The lenses are each disposed in an optical path formed between a corresponding one of the laser media and the diffraction grating, and the lenses superimpose the laser beams from the laser media on an incident surface of the diffraction grating such that the laser beams have an equal outer diameter.