Abstract: A buried layer semiconductor laser includes a mesa stripe comprised of a multi-layer structure having successively over a substrate at least a first semiconductor cladding layer of a first conductivity type, a semiconductor optical waveguide layer of said first conductivity type, a semiconductor active layer, a second semiconductor cladding layer of a second conductivity type, and a third semiconductor cladding layer of the second conductivity type. The sides of the semiconductor optical waveguide layer are covered by a first semiconductor burying layer of the second conductivity type and having a refractive index the same as or smaller than the refractive index of the optical waveguide layer. A second semiconductor burying layer of the first conductivity type covers the sides of the active layer, the second cladding layer and the third cladding layer. The lateral width of the active layer and the second cladding layer are smaller than the lateral widths of the other layers of the multi-layer structure.