A semiconductor laser, in which layers on both sides of the light emitting region are each formed by laminating a plurality of semiconductor layers of different energy band gaps and thicknesses smaller than 0.03 .mu.m. The thickness of the thin film layer of at least one kind of the semiconductor thin film layers of a thickness less than 0.03 .mu.m varies in dependence upon the layers remoteness from the light emitting region. The light emitting region and the layers on both sides of the light emitting region are each formed of a mixed crystal which consists of indium, gallium, arsenic and phosphorus, or indium, gallium, aluminum and arsenic and which has a lattice constant difference less than 0.3% relative to indium phosphide.