In a terraced substrate type semiconductor laser comprising a semiconductor substrate (11) having a step (T) on its principal face, an active layer (13) with an oblique central region (131), defined between two bendings, as stripe-shaped lasing region near the foot of the step part (T) of the substrate (11), and a clad layer (14) formed on the active layer (13),The device is characterized by comprising a current injection region (22) which is formed by diffusing an impurity, in a manner that a diffusion front corner (221) penetrate the clad layer (14) and contacts the oblique lasing region (131) thereby to form the current injection path (221) very narrow and closely to the central part of the stripe-shaped lasing region (13), thereby effectively confining the injected current to the lasing region (131) and hence attaining very low threshold current and very high external differential quantum efficiency.