A substrate manufacturing method capable of easily obtaining a thin magnesium oxide single crystal substrate is provided. A first step is performed which disposes a condenser for condensing a laser beam on an irradiated surface of a magnesium oxide single crystal member in a non-contact manner. A second step is performed which forms processing mark lines in parallel by irradiating the laser beam to the surface of the single crystal substrate under designated irradiation conditions to condense the laser beam into an inner portion of the single crystal substrate while moving the condenser and the single crystal substrate relative to each other in a two-dimensional manner. A third step is performed which forms new processing mark lines between the adjacent irradiation lines in the second step to allow planar separation, by irradiating the laser beam to the surface of the single crystal substrate under designated irradiation conditions to condense the laser beam into an inner portion of the single crystal substrate while moving the condenser and the single crystal substrate 20 relative to each other in a two-dimensional manner.