Method of manufacturing diamond substrate

A method of manufacturing a diamond substrate comprising:

• a step of placing a laser condensing unit configured to condense laser light so as to face an upper surface of a block of single crystal diamond as a main surface of said upper surface;
• and
• a step of forming a modified layer, which includes a processing mark of graphite, along the surface (111) of the single crystal diamond, at a predetermined depth from the upper surface of the block, and a crack extending from the processing mark to a surrounding area around the processing mark and a crack extending in a [1-10] direction from the processing mark to a surrounding area around the processing mark along the surface (111), by radiating the laser light on the upper surface of the block from the laser condensing unit and condensing the laser light inside the block, and moving the laser condensing unit and the block in a relative manner two-dimensionally,
• wherein in the step of forming a modified layer, a processing mark directing toward a lower surface by a first scan line directing in a [-1-12] direction and a processing mark directing toward the upper surface by a second scan line directing in a [11-2] direction are formed by the laser light,
• and wherein a cleavage occurs in a [1-10] direction and the modified layer expands due to an expansion of the processing mark directing toward the upper surface.