An object of the present invention is to provide an optical waveguide device and an optical waveguide module having the function to multiplex/demultiplex or couple/decouple optical signals and particularly to provide an optical waveguide device and an optical waveguide module for realizing low optical loss, reducing wavelength dependence and ensuring easier shape management in the manufacturing and inspection processes. The optical waveguide device and optical waveguide module are provided with a substrate, one or more first waveguides, a plurality of second waveguides, and a slab waveguide for connecting the first waveguides and the second waveguides. At a region where the second waveguides are connected to the slab waveguide, a plurality of waveguide paths are provided for respectively connecting the second waveguides. Width of the waveguide paths is constant. The width of the separation gap between waveguide paths progressively increases as it becomes further away from the slab waveguide.