The present invention provides a substrate formed at a low cost and having a controlled plate shape, an epitaxial layer provided substrate obtained by forming an epitaxial layer on the substrate, and methods for producing them. The method for producing the substrate according to the present invention includes an ingot growing step serving as a step of preparing an ingot formed of gallium nitride (GaN); and a slicing step serving as a step of obtaining a substrate formed of gallium nitride, by slicing the ingot. In the slicing step, the substrate thus obtained by the slicing has a main surface with an arithmetic mean roughness Ra of not less than 0.05 μm and not more than 1 μm on a line of 10 mm.