An optical modulation device includes: a crystal substrate having an electro-optic effect; an optical waveguide formed in the crystal substrate; an electrode formed on the crystal substrate, to apply an electric field to the optical waveguide; and a buried layer of low dielectric constant buried to avoid the optical waveguide, in at least one portion of a lower region of the electrode inside the crystal substrate, which is spaced from both of a front surface of the crystal substrate on which the electrode is formed and a rear surface thereof opposed to the front surface.