A β gallium oxide substrate manufacturing method for manufacturing a substrate from a workpiece formed of β gallium oxide includes a separation layer forming step of applying a laser beam of such a wavelength as to be transmitted through the β gallium oxide to the workpiece, with a focal point of the laser beam positioned at a predetermined depth from a front surface of the workpiece, to thereby form a separation layer including a modified part and a crack extending from the modified part, inside the workpiece, and a separation step of exerting an external force on the workpiece to thereby separate the substrate from the workpiece with the separation layer as a start point, after the separation layer forming step.