An object of the invention is to provide a Mach-Zehnder type optical modulator that can reduce wavelength chirp occurring in a modulated light, and also can reduce an optical loss with a small sized configuration. To this end, according to the present Mach-Zehnder type optical modulator, in a configuration where optical waveguides and a coplanar electrode are formed on a substrate having an electro-optical effect, a polarization inversed region is formed in a part of an interaction portion of the substrate, and a signal electrode is arranged above one of parallel waveguides in a polarization inversed region and is arranged above the other parallel waveguide in a non-inversed region. Furthermore, a surface area of the substrate positioned on both sides of each of the parallel waveguides is lowered to provide a ridge structure section, and a buffer layer is formed on a ridge side face thereof.