A 780 nm band semiconductor laser device has an InGaAsP well layer, phosphorous composition of which is 0.51 smaller than 0.55 to prevent spinodal decomposition in growing InGaAsP. A compressive strain of 0.65% less than 1% and more than 0.25% is introduced into the well layer to reduce threshold current thereof. Thus, the 0.78-.mu.m band semiconductor laser device having the InGaAsP well layer stably operates for a long time even in outputting a high optical power of 100 mW or more. A tensile strain of 1.2% is also introduced into barrier layers within the active region so as to compensate the stress due to the compressive strain of the well layer. As a result, the reliability of the semiconductor laser device is further increased during a high output operation.