An electroabsorption semiconductor optical modulator includes a light absorption layer for generating a modulated light beam by absorbing an incident light beam. A well layer in the light absorption layer, accumulates charge carriers generated by the light absorption layer. The charge carriers are guided and released from the well layer upon receipt of an incident excitation light beam having a wavelength corresponding to the bandgap energy of the well layer. The incident light beam is modulated by changes in absorption coefficient in response to an externally applied voltage. The modulator responds to a high-intensity incident light beam at high frequency, free from deterioration of extinction characteristics, and has good transmission characteristics.