An object of the present invention is to provide an optical waveguide element that effectively diffuses charges accumulated in a substrate, and suppress DC drift or temperature drift. The optical waveguide element includes a substrate having an electro-optical effect, optical waveguides formed in the substrate, a buffer layer (BF layer) formed on the substrate, and modulation electrodes (signal electrode and ground electrode) that are formed on the buffer layer and modulate optical waves propagating through the optical waveguides, a charge diffusion layer that diffuses charges generated in the substrate is formed between the substrate and the buffer layer, and the charge diffusion layer is electrically connected with a ground electrode constituting the modulation electrode.