A method for manufacturing a group III nitride semiconductor substrate includes a sapphire substrate preparation step S10 for preparing a sapphire substrate having, as a main surface, a {10-10} plane or a plane obtained by inclining the {10-10} plane at a predetermined angle in a predetermined direction; a heat treatment step S20 for performing a heat treatment over the sapphire substrate while performing a nitriding treatment or without performing the nitriding treatment; a buffer layer forming step S30 for forming a buffer layer over the main surface of the sapphire substrate after the heat treatment; and a growth step S40 for forming a group III nitride semiconductor layer, in which a growth surface has a predetermined plane orientation, over the buffer layer, in which at least one of a plane orientation of the main surface of the sapphire substrate, presence or absence of the nitriding treatment during the heat treatment, and a growth temperature in the buffer layer forming step is adjusted such that the growth surface of the group III nitride semiconductor layer has the predetermined plane orientation.