An electro-optic optical modulating apparatus comprises an optical waveguide, which has electro-optic effects, and at least a single pair of electrodes, which are located in the vicinity of the optical waveguide. A driving circuit applies a radio-frequency voltage across the electrodes. A modulating circuit modulates the amplitude of the radio-frequency voltage in accordance with a predetermined signal. The amount of a modulated optical wave is thus prevented from fluctuating due to a DC drift phenomenon.