A nitride semiconductor laser diode has a quantum well layer consisting of a mixed crystal of Alx1Iny1Ga1-x1-y1N (x1≧0.5, y1≧0 and 1−x1−y1≦0.5) in a group III nitride semiconductor multilayer structure having a major growth surface defined by a nonpolar plane. A cavity direction of the laser diode is perpendicular to a c-axis. The major growth surface of the group III nitride semiconductor multilayer structure may be defined by an m-plane. In this case, the cavity direction may be along an a-axis.