Abstract: The invention provides an optical modulator which suppresses the loss of a microwave which advances through an electrode and of light which propagates in a waveguide and makes the losses in individual arm waveguides substantially equal to each other to suppress the deterioration of the extinction coefficient to improve the transmission quality. The optical modulator includes a substrate having an electro-optical effect and having a ridge, first and second grooves and first and second banks formed thereon, a Mach-Zehnder optical waveguide, an electrode, and first and second recesses formed at symmetrical positions with respect to the ridge on the first and second banks, respectively. The optical modulator is applied, for example, to a transmission side apparatus for a long distance optical transmission system.