A method of manufacturing a diamond substrate includes: a step of placing a laser condensing unit 190 configured to condense laser light B so as to face an upper surface 10a of a block 10 of single crystal diamond; and a step of forming a modified layer 20, which includes a processing mark 21b of graphite and a crack 22b extending along a surface (111) around the processing mark 21b, along the surface (111) of the single crystal diamond at a predetermined depth from an upper surface of the block by radiating the laser light B on the upper surface 10a of the block 10 from the laser condensing unit 190 under predetermined conditions and condensing the laser light B inside the block 10, and moving the laser condensing unit 190 and the block 10 in a relative manner two-dimensionally.