Abstract: A semiconductor laser diode of pn double hetero junction type which comprises a first active layer formed on a substrate for imparting a main oscillation, a second active layer adjacent to the first active layer in such a manner that at least one of the first and second active layers is formed of a multi quantum well or single quantum well structure, an intermediate clay layer interposed between the first active layer and the second active layer for preventing duplication of wave function of confined electrons of the respective active layers, and a graded refractive index distribution region (GRIN-SCH structure layer) provided on at least one of under the first active layer and over second active layer to reduce the refractive index thereof remotely from the active layers.