PURPOSE: To make even a laser beam with a high peak power convertible to an intensity distribution of a stable uniformized transverse mode.

CONSTITUTION: A laser beam 10a is converged through lenses 2, 3, and is made incident on a hollow waveguide 5 at a lens 4. The lens 4 is provided with a negative value in the back focal distance; its imaginary condensed point is P1; the diffused laser beam 10c travels reflecting inside the hollow waveguide 5, is outputted as a laser beam 10d with a uniformized intensity distribution of transverse mode, condensed through condenser lenses 6, 7; the workpiece 8 is then irradiated with the laser beam and the surface treatment for the workpiece is performed. At this time, with the lens 4 provided with the negative value in the back focal distance, there is no condensed point inside the hollow waveguide 5, and therefore, the output of the laser beam is not reduced or unstabilized through the ionization of air.