Abstract: A narrow-band laser apparatus comprising a laser generating section having laser activity and capable of generating laser beam; bandwidth narrowing section which is provided on one end side of the laser generating section on the optical axis of the laser beam generated in the laser generating section to narrow the bandwidth of the laser beam by means of at least one angel-dispersion wavelength selection element, fold back the laser beam in the narrowed band and output the folded laser beam toward the laser generating section; beam folding section which is provided on the other end of the laser generating section on the optical axis of the laser beam generated in the laser generating section to fold back the input laser beam and reflect the folded laser beam into the laser generating section; and a laser beam branching unit which is provided between the laser generating section and the bandwidth narrowing section to permit part of the laser beam input from the laser generating section to pass therethrough into