An optical waveguide device 1A has an optical waveguide substrate 10A and a supporting substrate 6 supporting the substrate 10A. The substrate 10A has a main body 4 made of an electrooptic material and having first main face 4a and a second main face 4b, an optical waveguide 3 formed in or on the main body 4 and an electrode 2A, 2B or 2C formed on the side of the first main face 4a of the main body 4. The supporting substrate 6 is joined with the second main face 4b of the main body 4. A low dielectric portion 11 with a dielectric constant lower than that of the electrooptic material is formed in the supporting substrate 6.