An optical waveguide is formed of a stack of electro-optic effect films. A stress-relief layer is formed between a substrate and the optical waveguide. The stress-relief layer is comprised of a metal material having a thermal expansion coefficient of 10.times.10.sup.-6/.degree. C. or higher, for example, a metal material whose major component is one of Au, Ag, and an alloy thereof, and has a function of relieving a binding force to the optical waveguide ascribable to the substrate.