[Problem] The purpose of the present invention is to provide a highly efficient, high quality group-III nitride semiconductor element, and to provide a novel aluminum nitride substrate (aluminum nitride single crystal substrate) for fabricating the group-III nitride semiconductor element. [Solution] A substrate comprising aluminum nitride, wherein the aluminum nitride substrate has on at least a surface thereof an aluminum nitride single-crystal layer having as a principal plane a plane that is inclined 0.05°to 0.40° in the m-axis direction from the (0001) plane of a wurzite structure.