Abstract: An ultraviolet range laser comprising a vacuum vessel having an open end portion hermetically sealed with an output mirror glass substrate, which is featured in that the output mirror glass substrate is formed of a glass material which is capable of allowing a laser beam of 337 nm in wavelength to pass therethrough at a transmittance of 80 to 96% as measured when the thickness of the glass substrate is set to 5 mm and whose thermal expansion coefficient falls in the range of 44.times.10.sup.-7 /.degree.C. to 55.times.10.sup.-7 /.degree.C. at a temperature of 0.degree. C. to 300.degree. C., and in that a metallic sealing ring is hermetically attached to the open end portion of vacuum vessel and the hermetical sealing of the open end portion of vacuum vessel with the output mirror glass substrate is achieved by hermetically adhering the output mirror glass substrate onto the metallic sealing ring via a low melting point solder glass interposed therebetween.