An optical modulator which utilizes the Stark effect according to which the absorption spectra change if an electric field is applied to the excitons. A thin film of a suitable thickness composed of a semiconductor and an insulator or composed of either one of them, is formed between a group of electrons and a group of positive holes that constitute excitons, so that the excitons are stabilized. The optical modulator performs the modulation at high speeds maintaining a high efficiency.