An optical modulator 1C has a substrate 2 of an electro-optic material and having first and second main faces, an optical waveguide formed on the substrate 2 and having first branched part 5 and a second branched part 3, and ground electrodes 4A, 4C and a signal electrode 4B provided on the side of the first main face of the substrate. The first branched part 5 and second branched part 3 are provided between the edges of the ground electrodes 4A, 4C and the edge of the signal electrode 4B, respectively. Microwave electric fields are applied onto interacting parts of the first branched part 5 and second branched part 3, respectively, to modulate light propagating in the first branched part 5 and second branched part 3, respectively. The integral values of field intensities over interaction lengths with electrodes in the first and second branched parts are different from each other so that a predetermined chirp amount is obtained.