A laser apparatus includes an oscillator that outputs laser light, an amplifier, a front optical system and a rear optical system that are disposed at positions where the front and rear optical systems face each other with a chamber sandwiched therebetween and constitute a ring resonator having a first optical path and a second optical path, and first plane parallel substrates disposed on the first optical path or the second optical path. The first optical path is an optical path along which the front optical system outputs the laser light. The second optical path is an optical path along which the rear optical system outputs the laser light. The first plane parallel substrates translate the first optical path and the second optical path, respectively, in the directions in which the first and second optical paths approach each other on the side facing the chamber.