Disclosed is an electro-absorption type semiconductor optical modulator utilizing the Quantum Confinement Stark Effect, in which a quantum well structure introduced in its optical absorption layer is arranged to have a potential structuresuch that one of the electron affinity and the energy of the top of the valence band increases in the laminating direction, while the other decreases, thereby canceling the built-in field. It is intended to lower the drive voltage and to enhance an on/off ratio (extinction ratio). Thus, the absorption peak becomes narrow at a no bias state to attain a low drive voltage and an enhanced extinction ratio.