An optical modulator with lowered chirping without deterioration of extinction characteristic. An intermediate layer is interposed between a well layer and an n-side barrier layer and tensile strain is produced in the well layer. A relationship EwEmEb is established between a band gap energy Eb (eV) of the barrier layer, the band gap energy Ew (eV) of the well layer, and the band gap energy Em (eV) of the intermediate layer. Another type of tensile strain quantum well structure having four kinds of layers may be employed by adding another layer to the tensile strain quantum well structure having three layers. The band gap energy of an additional layer is between the band gap energies of the well layer and the intermediate layer or between the band gap energies of the intermediate layer and the barrier layer.