A low-cost high-property optical semiconductor element for a long wavelength is provided, using a GaAs substrate. The optical semiconductor element comprises a substrate of GaAs having a first surface and a second surface opposite to each other, a buffer layer of In.sub.jGa.sub.1-jAs.sub.1-kN.sub.k (0.ltoreq.j.ltoreq.1, 0.002.ltoreq.k.ltoreq.0.05) formed on the first surface of the substrate, a first conductive type clad layer formed on the buffer layer, an active layer formed on the first conductive type clad layer and comprising a well layer of In.sub.zGa.sub.1-zAs (0.ltoreq.z.ltoreq.1), the well layer having a smaller bandgap than the first conductive type clad layer, the active layer having a thickness of more than its critical thickness for the substrate based upon equilibrium theories, and a second conductive type clad layer formed on the active layer and having a larger bandgap than the well layer.