An optical waveguide structure has a single port on one input/output side, a plurality of ports on another input/output side and an S-curve optical waveguide portion arranged on the outermost side of the waveguide structure and including a first circular arc optical waveguide portion and a second circular arc optical waveguide portion connected thereto. The waveguide portion also has a third circular arc optical waveguide portion extending from a first splitting/coupling point side in the first circular arc waveguide portion toward the plural/ports side and having a curvature inverted relative to the first circular arc optical waveguide portion. At the first splitting/coupling point, a tangential line of the first circular arc optical waveguide portion and a tangential line of the third circular arc optical waveguide portion are parallel to and spaced from each other.