PURPOSE: To stabilize a transverse mode and a laser output power even under any vibration, external force, and temperature variations by detecting elongation of supporter means and controlling cooling media fed to each supporter means on the basis of a detected signal.

CONSTITUTION: There is employed for a supporter rod 7 one having high rigidity quality. Since however the present supporter rod 7 comprises a material different from invar having less expansion with respect to temperature, a strain gauge 10 is sticked to the supporter rod 7 at several locations. A signal from the strain gauge 10 is incorporated into a control device 11. The control device 11 evaluates the elongation of each supporter rod 7 and such an amount of cooling water that the amounts of the elongations of all supporter rods 7 are equal to each other. Hereby, the amount of cooling water to each supporter rod 7 is adjusted by a valve 13 of which valve travel can be adjusted with the aid of a motor 12 on the basis of the evaluated amounts of water. Thus, even if any vibration, external force, and temperature variation are produced on a resonant structure, the laser light is stabilized in its transverse mode and output power.