Abstract: A laser beam having desired properties is output at desired timings. A laser apparatus is a laser apparatus for use with an extreme ultraviolet light generating apparatus that generates extreme ultraviolet light at a repetition frequency which is set in advance, and may be equipped with: a semiconductor laser that outputs a laser beam when a trigger signal is input thereto; an optical switch that switches between a state in which the laser beam passes therethrough and a state in which the laser beam does not pass therethrough, provided along the optical path of the laser beam; and a control unit configured to output the trigger signal to the semiconductor laser at a frequency which is an integer multiple of the repetition frequency, and to control the optical switch such that the laser beam passes therethrough at the repetition frequency.