A GaN substrate is stored within an atmosphere in which the oxygen concentration is not greater than 15 vol. % and the water-vapor concentration is not greater than 20 g/m3. The GaN substrate (1) has a planar first principal face (1m), and in an arbitrary point (P) along the first principal face (1m) and separated 3 mm or more from the outer edge thereof, the GaN substrate's plane orientation has an off-inclination angle Δα of −10° or more, 10° or less with respect to the plane orientation of an arbitrarily designated crystalline plane (1a) that is inclined 50° or more, 90° or less with respect to a plane (1c), being either the (0001) plane or the (000 1) plane, through the arbitrary point. This enables storing GaN substrates whose principal-face plane orientation is other than (0001) or (000 1), making available GaN substrates with which semiconductor devices of favorable properties can be manufactured.