A first optical waveguide, a second optical waveguide, a first electrode, and a second electrode are integrated on a substrate. An optical modulator is provided with a clock signal generator for generating an RZ signal by applying a clock signal to either the first or second electrode, and an NRZ data signal generator for supplying an NRZ data signal to the remaining electrode. Thus, the space required by the optical modulator is reduced while tolerance of the same is improved, thus reducing costs for constructing the optical modulator.