A curved optical waveguide comprising a core and a clad, characterized in that: the core shape of the curved optical waveguide has no reversal of a curvature on a halfway; and curvatures at both ends of the curved optical waveguide gradually approach zero. A curved optical waveguide comprising a core and a clad, characterized in that: the core shape of the curved optical waveguide has no reversal of a curvature on a halfway; a curvature at one end of the curved optical waveguide gradually approaches zero; and a radius of curvature at other end is finite. An optical waveguide comprising such a curved optical waveguide and an optical waveguide having a different core shape optically connected with the former, and an optical device using such a curved optical waveguide.