To split precisely in following the irradiation locus of laser beam and not to generate a microcrack on the edge surface, rear surface and side surface of substrate to be split in cutting a substrate.

In a substrate splitting method to split from one end of a substrate 1 to the other end by traveling the substrate l while irradiating on the surface of substrate to be worked made of non-metallic material with laser beam by causing assist gas to flow along the transfer direction of a carbon dioxide gas laser beam 3, by closely adhering dummy substrates 2-1, 2-2 made of non-metallic material on one end and the other end of the substrate 1 to be worked, the substrate l to be worked is cut together with the dummy substrates 2-1, 2-2.