An excimer laser comprises a band-narrowing optical system (6) having an advantage that the oscillation center wavelength hardly varies with temperature and provided with a Littrow-mounted reflection-type diffraction grating (3) and a beam-expanding optical system (5) disposed on the light-incidence side of the reflection-type diffraction grating (5). The beam expanding optical system (5) is composed of one or more beam expanding prism (11, 12) made of fluorite and one or more beam-expanding prism (13) made of synthetic quartz glass. The rate of change of the oscillation center wavelength with respect to temperature change is made small by considering the number, arrangement, and direction of prisms (11, 12, 13).