An optical modulation device includes: an optical splitter for splitting input light into a first input beam and a second input beam; an optical intensity modulator for modulating the intensity of the first input beam in response to a modulating signal; a variable optical phase shifter for shifting the phase of the second input beam; and an optical combiner for combining an output beam of the optical intensity modulator and an output beam of the variable optical phase shifter into a combined beam and outputting the combined beam. The amount of phase shift produced by the variable optical phase shifter is externally controlled.