The present techniques provide a method for producing a Group III nitride semiconductor single crystal that is designed to grow a semiconductor single crystal with high reproducibility. The method for producing a Group III nitride semiconductor single crystal comprises adding a seed crystal substrate, Ga, and Na into a crucible, and growing a Group III nitride semiconductor single crystal. In the growth of the Group III nitride semiconductor single crystal, a measuring device is used to detect the reaction of Ga with Na. Ga is reacted with Na with the temperature of the crucible adjusted within a first temperature range of 80° C. to 200° C. After the measuring device detected the reaction of Ga with Na, the temperature of the crucible is elevated up to a growth temperature of the Group III nitride semiconductor single crystal.