An optical waveguide, on account of its ability to apply phase resonance of a wavelength and of a first and second triangular plate-like spot size converter members formed of the same material as a core material and being arranged and formed in a substantially symmetrical structure, can promote shortening of the waveguide length and contrive to reduce the size of the optical waveguide itself. Further, an optical waveguide having excellent spot size conversion efficiency can be obtained even in a reduced size.