Provided is a laser device that includes a laser chamber in which a pair of discharge electrodes are disposed; a line narrowing optical system including a grating disposed in a position outside the laser chamber; a beam expander optical system that increases a diameter of a light beam, outputted from the laser chamber and traveling toward the grating, in a first direction parallel to a discharge direction between the discharge electrodes and in a second direction orthogonal to the discharge direction; and a holding platform that is formed as a component separate from the laser chamber and the grating, holds the beam expander optical system, and forms along with the beam expander optical system a beam expander unit.