An optical waveguide device has an optical waveguide substrate and a supporting body for supporting the substrate. The substrate has a main body made of an electro-optic material and having a main face and an opposing face, optical waveguides, and electrodes for applying an electrical signal on the optical waveguides. At least a part of the opposing face of the supporting body opposing the substrate is covered with a conductive layer. It is thus possible to reduce the resonance due to substrate radiation leakage of the microwave signal into the whole of the optical waveguide substrate and supporting body.