A wideband semiconductor electro-absorption optical modulator including a semiconductor core shorter in absorption-peak wavelength than a wavelength of optical signal, and an electrode for applying an electric signal to absorb the optical signal by shifting the absorption-peak wavelength to a long wavelength region when a voltage is applied, wherein an electric signal input port and an electric signal output port are disposed so that the electrode is constructed in the form of a traveling-wave electrode, and a total thickness of non-doped layers including the semiconductor core is reduced to decrease a driving voltage. Degradation of optical modulation bandwidth and reflection characteristics of the electric signal caused by mismatching of characteristic impedance to an outer circuit are reduced by decreasing an interaction length of the electric signal and the optical signal. Further, mismatching of characteristic impedance is corrected by adjusting a doping concentration of a p-type or n-type doped layer located above or beneath the semiconductor core.