Diamond substrate manufacturing method

A diamond substrate manufacturing method for manufacturing a diamond substrate from a workpiece including diamond and having a first surface and a second surface on a side opposite to the first surface, the diamond substrate manufacturing method comprising:

• a focal point positioning step of positioning a focal point of a processing laser beam of such a wavelength as to be transmitted through diamond to a depth from the first surface, the depth corresponding to a thickness of the diamond substrate to be manufactured;
• a peeling layer forming step of applying the processing laser beam to the workpiece while relatively moving the focal point and the workpiece, to form a peeling layer including graphite sections formed through graphitization of portions inside the workpiece and cracks extending from the graphite sections;
• and
• a peeling step of applying a peeling laser beam of such a wavelength as to be transmitted through the diamond and absorbed in graphite to the entire peeling layer of the workpiece simultaneously, to heat the graphite sections to such a temperature that the graphite sections react with oxygen in an atmosphere, thereby causing at least a part of the graphite sections to react with oxygen in the atmosphere to generate carbon dioxide and to form gaps, and peeling off the diamond substrate from the workpiece with the gaps as a starting point, after the peeling layer forming step is carried out, wherein the peeling off the diamond substrate is a dry process without application of ultrasonic vibrations.