PROBLEM TO BE SOLVED: To obtain a laser with narrow band operating over a long term while improving the wavelength stability by combining a beam expander with a line narrowing system for correcting the wave front distortion caused by a tubular mirror. SOLUTION: An output coupler comprises a partial reflection tubular convex mirror 68 and the output from a gain medium 65 is made narrow in a line narrowing module. The line narrowing module comprises a prism beam expander 70 for enlarging a beam in one direction perpendicular to the axis of the tubular mirror 68, a turning total reflector 72, and a lattice 74. The lattice 74 has an adjustable curve and adjusted to correct a wave front distortion caused by the tubular mirror 68. The lattice is curved by a compression spring, and a threaded micrometer push bolt 76 for applying a compressive force to the inside of legs 78, 80 extending from the opposite ends of the lattice 74. Apertures 82, 84 limit the cross-sectional dimensions of a beam.