Abstract: The problems addressed by the present invention lies in providing a Group III nitride semiconductor substrate having a principal plane on which high-quality crystals can be grown and also providing a method for producing a Group III nitride semiconductor substrate capable of obtaining a crystal which has few stacking faults and in which stacking faults in directions parallel to the polar plane in particular have been greatly suppressed. The problem is solved by means of a Group III nitride semiconductor substrate having a plane other than a C plane as a principal plane, wherein a ratio (W1/W2) of a tilt angle distribution W1 of the principal plane in the direction of a line of intersection between the principal plane and the C plane to a tilt angle distribution W2 of the principal plane in a direction orthogonal to the line of intersection is less than 1.