Laser lithography is an optical forming method which heaps up thin layers solidified by laser beam. Since plastic models which are made by laser lithography process have ledges on their surfaces, reducing ledges is required to use them as master models for jewelry. A diagonal irradiation method for reducing ledges is presented. The diagonal irradiation method improves surface roughness by means of heap up diagonal shape solidified by diagonal laser irradiation. Diagonal solidifying behavior was clarified by theoretical calculation. As the result of an experiment, average surface roughness of 10μm in steep slope area and improvementof out of roundness was obtained by the diagonal irradiation method.