An optical waveguide device 1 has an optical waveguide substrate 19, a supporting body 2 for supporting the substrate 19 and a joining layer 3 for joining the substrate 19 and the supporting body 2. The substrate 19 has a flat plate-shaped main body 4 made of an electro-optic material with a thickness of 30 m or smaller and having first and second main faces 4a and 4b opposing each other, an optical waveguide provided on the side of the first main face 4a of the main body 4, and electrodes 7A to 7C provided on the side of the first main face 4a of the main body 4. The joining layer 3 joins the supporting body 2 at a joining face 4d and the second main face 4d of the main body 4. The joining face 2a of the supporting body 2 is substantially flat. Alternatively, the joining layer 3 has a thickness of 200 m or lower.