A laser apparatus capable of emitting laser beams of a plurality of different wavelengths includes: a solid-state laser medium which emits light of a plurality of different peak wavelengths; a resonance optical system which resonates the emitted light of the plurality of different peak wavelengths and converts respective light to oscillate the laser beams of the plurality of different wavelengths; and a 1/4 wave plate for a wide band, which is placed in the resonance optical system and has a property of providing a uniform phase difference to the light of the plurality of different peak wavelengths to be converted.