PURPOSE: To position a mode-varying means to an optical axis positively, and to change a mode continuously.

CONSTITUTION: An aperture 33 as a mode varying means having an opening section 34 for passing laser beams is mounted movably along the optical axis 35 of laser beams between a reflecting mirror 12 and an output mirror constituting a resonator. A higher mode is obtained because laser beams passing through the opening section 34 are hardly limited by the aperture 33 under the state, in which the aperture 33 is moved to the left side in the figure, and a lower mode is acquired because laser beams are much limited by the aperture 33 in response to the shifting to the right side of the aperture 33.