A method for producing an aluminum nitride single crystal substrate, the method including: i) preparing a first base substrate consisting of a first aluminum nitride single crystal; ii) growing a first aluminum nitride single crystal layer having a thickness of no less than 500 µm over a main face of the first base substrate, to obtain a layered body; iii) cutting the first aluminum nitride single crystal layer of the layered body, to separate the layered body into a second base substrate and a first part of the first aluminum nitride single crystal layer, the second base substrate including the first base substrate and a thin film layered thereon, the thin film being a second part of the first aluminum nitride single crystal layer; iv) polishing a surface of the thin film in the second base substrate, to obtain a third base substrate consisting of a second aluminum nitride single crystal; and v) growing a second aluminum nitride single crystal layer over the polished surface of the third base substrate.