There may be provided a laser amplifier including: a chamber containing a laser medium; a first window provided on the chamber, and configured to allow a laser light beam inputted from outside of the chamber to enter the chamber; an excitation unit configured to amplify, by exciting the laser medium, the laser light beam that has entered the chamber; a second window provided on the chamber, and configured to allow the laser light beam that has been amplified by the excitation unit to exit from the chamber to the outside; a mirror provided on a laser light path between the first window and the second window; and a wavelength selection film provided on one or more of the first window, the second window, and the mirror, and configured to suppress propagation of light beams of one or more suppression target wavelengths different from a desired wavelength.