PURPOSE: To lessen such a phenomenon that the electric field to be applied to an optical waveguide from an electrode becomes unstable due to a buffer layer by composing the buffer layer of at least two buffer layers which are specified respectively.

CONSTITUTION: A first and a second buffer layers 3, 5 are formed on an optical waveguide 2 and an electrode film 4 is put in the part where the optical waveguide 2 is divided. The buffer layers 3, 5 between the optical waveguide 2 and the electrode film 4 have to satisfy three conditions that they have lower refractive index than the optical waveguide 2, they do not absorb evanescent wave of light transmitted into in the optical waveguide 2, and they have high resistance to prevent the occurrence of current leakage between the electrode film 4 and the optical waveguide 2. The first buffer layer 3 is formed from a material having a smaller refractive index than the optical waveguide 2 and having wavelength property not to absorb the light transmissive in the optical waveguide 2 and the second buffer layer 5 which is positioned nearer to the electrode 4 side than the first buffer layer 3 is formed from an insulating material.