To provide an apparatus which converts a laser beam having a Gaussian distribution and other nonuniform power distributions into a uniform power distribution beam of a top hat type.

A diffraction point where the power is equally distributed in a specified range is formed and is defocused, by using a diffraction type optical element and a condenser lens, to obtain the optical quantity distribution of the top hat type within this range. The phase pattern of the diffraction type optical element (DOE) is previously determined in such a manner that the beam power at the lattice point within a certain range is made the same. The laser beam having the nonuniform distribution is cast to the diffraction optical element and the transmitted light thereof is condensed by the condenser lens. The beam emitted from the condenser lens is made into the uniform optical quantity distribution on an objective surface by defocusing to slightly shift the objective surface forward and backward from the focal plane.