Disclosed is a method for producing a group III nitride semiconductor crystal wherein an alloy (11) containing at least a group III metal element and an alkali metal element is put in a reaction container (1) and a nitrogen-containing substance (14) is introduced into the reaction container (1), so that the nitrogen-containing substance (14) is dissolved in an alloy melt (13) of the molten alloy (11) and there is grown a group III nitride semiconductor crystal (15). By this method, there can be efficiently obtained a group III nitride semiconductor crystal (15) having a low light absorption coefficient. Also disclosed is a group III nitride semiconductor device having a high luminous intensity.