An optical waveguide element includes a substrate showing electrooptical effects and having a pair of major planes opposed with each other, an optical waveguide formed on one major plane of the substrate, and an electrode for applying a modulation signal to an optical wave transmitted through the optical waveguide. In the optical waveguide element mentioned above, there are a thin thickness portion having a relatively thin thickness formed to a portion of said substrate at least positioned correspondingly to said electrode; and a buffer layer formed between said substrate and said electrode, wherein an impedance conformity with the modulation signal applied to said electrode is performed by controlling a thickness of said thin thickness portion.