PURPOSE: To improve the amplification efficiency and facilitate long distance propagation by a method wherein the difference in beam divergence angle created by the difference between the directions in the cross section of a laser beam is reduced and the superposition state in an amplifier is improved.

CONSTITUTION: An optical system is mainly composed of a collimator lens 8 which converts a laser beam emitted from a laser diode 7 into a parallel light and a prism pair 9 which expands the short axis side of the beam converted into the parallel light. The laser beam cross section is made to be approximately circular by the optical system and the difference in beam divergence angle is eliminated. Thus, the difference between the divergence angle of the long axis direction and the divergence angle of the short axis direction in the beam is reduced. With this constitution, approximately same superposition state can be obtained in the long axis direction and the short axis direction so that the amplification efficiency can be improved. Moreover, as the difference in divergence angle is minimized, the long distance propagation can be realized.