The present invention provides an optical waveguide device having a configuration such that the optical waveguide is folded back on an end area of the optical waveguide substrate to widen the modulation band. The optical waveguide device 1A includes a substrate body 2 made of an electro-optical material, optical waveguide 6, and modulation electrodes 3, 4 and 5 for applying a voltage on the optical waveguide 6. The optical waveguide 6 includes first primary areas 6e and 6f, a first curved area 7A, first folding-back areas 6g and 6h provided between the first curved area 7A and a folding-back point 8, second primary areas 6m and 6n, a second curved area 7B, and second folding-back areas 6j and 6k provided between the second curved area 7B and the folding-back point 8. At least a part of the signal electrode is provided in a folding-back region extending from the first curved area 7A and the second curved area 7B to the folding-back point 8.