The transmittance of an etalon relative to an oscillating line having the wavelength of 1061.6 to 1062.2 nm by an Nd:YAG laser medium 3 is set to 0.9 or lower (the maximum value of the transmittance is 1.0). The oscillating line of the wavelength of 1061.6 to 1062.6 nm is damped by the etalon so that a peak corresponding thereto does not substantially appear in an outputted laser beam. Accordingly, a longitudinal mode can be preferably changed to a single mode and an output can be stabilized.