In order to separate a material layer from a substrate at the boundary face between the substrate and the material layer, a laser light is applied to a workpiece from the substrate side through a mask, the work having the material layer formed on the substrate. The laser beam is split into a plurality of small area laser light by the mask 44, and two or more irradiation regions are formed on the workpiece. Adjacent irradiation regions are separated from each other, and an edge part of each irradiation region and an edge part of an adjacent irradiation region, which extend in a direction parallel to the relative moving direction of the workpiece, are arranged such that the edge of the irradiation region and the edge of the adjacent irradiation region are sequentially overlapped each other as the work is moved.