To provide a laser beam machining method by which a brittle material substrate such as a sapphire substrate is divided into pieces without causing scattering objects and even a comparatively thick substrate is easily divided into pieces with a reduced scanning frequency.

The laser beam machining method is for dividing a brittle material substrate into pieces by irradiating it with a pulse laser beam and includes a first and a second steps. In the first step, the brittle material substrate is irradiated with a pulse laser beam having a prescribed repeating frequency so that a converging point is situated inside the brittle material substrate to form a modified layer therein. In the second step, the pulse laser beam is scanned along a planned dividing line. Then, through the above steps, cracks are developed for the thickness t of the brittle material substrate in a length of 15-55% of the thickness t from the modified layer to the surface of the substrate.