An electro-optic modulator includes a substrate made of a material which has an electro-optic effect and a pyroelectric effect. In the substrate, an optical waveguide is formed to have at least a pair of optical paths. On the substrate and on the optical waveguide, a transparent buffer layer is formed to cover the optical waveguide. On the buffer layer, first and second electrodes are formed so that the first and the second electrodes are arranged to cause refractive index changes in the pair of optical paths in response to electrical fields surrounding the electro-optic modulator. The buffer layer is a mutual diffusion layer. The mutual diffusion layer is made from laminated films having at least one transparent insulator film and at least one transparent conductor film but has no clear boundary between the transparent insulator film and the transparent conductor film.