Abstract: A quantum well structure is provided which is capable of efficiently confining electrons and holes in a quantum well layer. The quantum well structure includes a first cladding layer, a second cladding layer, and a plurality of quantum well layers and one or more barrier layers each disposed between the first cladding layer and the second cladding layer. The quantum well layers and the barrier layers are laminated in an alternating manner. The quantum well layers include at least two selected from the group consisting of a layer having tensile strain, a layer having no strain, and a layer having compressive strain. The thickness of each of the quantum well layers is selected so that the energy difference in each of the quantum well layers between the ground quantum state of an electron at the conduction band and the ground quantum state of a hole at the valence band is substantially the same.