The present invention provides a manufacturing method that makes it possible to manufacture a substrate that is formed of high-quality Group III nitride crystals alone and has less warping. A Group III nitride layer (a seed layer 12 and a selective growth layer 15) including gaps is formed on a substrate (a sapphire substrate 11). In an atmosphere containing nitrogen, the surface of the Group III nitride layer is brought into contact with a melt containing alkali metal and at least one Group III element selected from gallium, aluminum, and indium, and thereby the at least one Group III element and the nitrogen are made to react with each other to grow Group III nitride crystals (GaN crystals 16) on the Group III nitride layer. Thereafter, a part including the substrate and a part including the Group III nitride crystals are separated from each other in the vicinities of the gaps.