Abstract: A semiconductor laser device includes a semiconductor substrate, upper and lower cladding layers each having a composition lattice-matching with the semiconductor substrate, which lower cladding layer is disposed on the semiconductor substrate, and an active layer having a quantum well structure interposed between the upper and lower cladding layers. The active layer includes alternating well layers and barrier layers and outermost guide layers, and these are arranged so that two of the barrier layers and the guide layers sandwich each well layer. The guide layers lattice-match with the upper and lower cladding layers, and the well layer has a lattice constant different from lattice constants of the two layers sandwiching the well layer, thereby applying a tensile strain to the well layer. The tensile strain reduces the effective mass of holes in the quantum well active layer, resulting in low threshold current, high efficiency, and high-power output.