An optical device having a substrate having an electro-optical effect, an optical waveguide formed in the substrate, and an electric field emission electrode, and an electric field convergence electrode to which an electric field emitted from the electric field emission electrode converges. The optical waveguide is formed such that light propagating through the optical waveguide passes through an area under the electric field emission electrode or an area under the electric field convergence electrode and then passes through a remaining electrode. As a result, even when an interaction length is increased, the optical device acquires (increases) a modulation index appropriate to the increase in interaction length, thereby diminishing a drive voltage.