An optical waveguide device that is implemented between a slot waveguide and a rectangular waveguide includes: a tapered portion and a separation portion. The tapered portion includes first and second cores that are respectively coupled to cores of the slot waveguide and formed in parallel each other. The separation portion includes third and fourth cores that are respectively coupled to the first and second cores. Cross-sectional areas of the first and second cores are substantially equal each other at an input end. The cross-sectional area of the first core is larger than that of the second core at an output end. A shape of a cross section of the first core changes continuously between the input end and the output end in the tapered portion. A spacing between the third core and the fourth core is continuously extended in the separation portion.