An object of the present invention is to improve the modulation efficiency of an optical modulator in a high frequency band while satisfying the velocity matching condition. An optical modulator is provided having an optical waveguide for propagating light, an electrode for applying a voltage on the waveguide for modulating the light, a signal source electrically connected to the electrode and a terminating resistance electrically connected to the electrode. The signal source has a characteristic impedance Zi and the terminating resistance has an impedance Zl satisfying the formula (Zi