The present invention provides a small optical waveguide structure capable of converting the spot size of light, and capable of reducing the conversion loss when compared under the condition of the same waveguide length and performing an optical conversion with high efficiency. An optical waveguide structure (100) includes a base waveguide (110) including a taper section (111) whose width becomes continuously narrower from one side toward another side, and a narrow-width section (112) that is consecutively connected to a narrow-width side of the taper section (111) and extends toward the another side. In the optical waveguide structure (100), at least three-layered upper waveguides (121 to 123) each of which has a planar shape smaller than the taper section (111) and includes a planar-view-roughly-wedge-shaped section whose width becomes continuously narrower from the one side toward the another side at least on a tip side are stacked above the taper section (111) of the base waveguide (110) in such a manner that the planar shape becomes successively smaller from the base waveguide side (110).