PURPOSE: To allow polarization inversion with the voltage as low as not to induce substrate destruction at the time of executing period polarization inversion by a voltage impression method and to make it possible to maintain sufficient strength by making the substrate thickness of a region where a waveguide is formed thinner than the thickness in other regions.

CONSTITUTION: This optical device is constituted by forming the waveguide 18 on a substrate 17 having a nonlinear optical effect and forming the period polarization inversion structures within the substrate 17 including the waveguide. The thickness of the substrate in the region where the waveguide 18 is formed is made thinner than the thickness in the other region. Namely, the band-shaped waveguide 18 is formed on one main surface side of the substrate 17 having the nonlinear optical effect and a groove 13 is formed in the region along the waveguide 18 on the other main surface side. As a result, the thickness of the substrate 17 is made thinner in the region where the waveguide 18 is formed and, therefore, the breaking electric field intensity of the substrate 17 is improved and the polarization inversion voltage is dropped. In addition, the easy execution of the polarization inversion is made possible, the polarization inversion regions are obtainable with good accuracy and moreover, the maintenance of the substrate strength is possible as well.