A laser apparatus according to an aspect of the present disclosure includes a master oscillator configured to emit a laser beam, an amplifier including an optical resonator and configured to amplify the laser beam emitted by the master oscillator in the optical resonator, and a phase shift structure disposed on an optical path between the master oscillator and the amplifier at a position closer to the amplifier than a middle point of the optical path. The phase shift structure includes a plurality of cells having different phase shift amounts for the laser beam. The cells have a disposition interval of 80 m to 275 m inclusive.

A laser apparatus includes an oscillation stage laser configured to generate and output a pulse laser beam, and an amplification stage laser configured to amplify the pulse laser beam output from the oscillation stage laser. The amplification stage laser includes an optical resonator configured to be a transfer optical system, a first discharge electrode pair and a second discharge electrode pair separated from each other in a first direction intersecting an optical path of the optical resonator, the first and second discharge electrode pairs being disposed across the optical path and configured to alternately discharge, and a slit disposed at a transfer position of the transfer optical system and configured to limit a beam size in a second direction orthogonal to the first direction of the pulse laser beam amplified by discharge of one of the first and second discharge electrode pairs.

A gas laser device configured to amplify, using an amplifier, laser light output from a laser oscillator and output the laser light. The amplifier includes a chamber device including a pair of discharge electrodes, and being configured to amplify the laser light by a voltage being applied between the pair of discharge electrodes; a resonator causing the laser light output from the chamber device to resonate; and a beam expander. The resonator includes an output coupling mirror causing a part of the laser light to be transmitted therethrough, and another part to be reflected back into the chamber device, and the beam expander including a convex mirror reflecting the laser light to expand a beam width of the laser light, and a concave mirror reflecting the laser light toward the output coupling mirror to collimate the laser light so that the expanded beam width of the laser light becomes constant.