An integrated optical waveguide has a first optical waveguide, a second optical waveguide, and a groove. The second optical waveguide is coupled to the first optical waveguide and has a refractive index that is different from the first optical waveguide. The groove is disposed so as to traverse an optical path of the first optical waveguide and is separated from an interface between the first optical waveguide and the second optical waveguide by a predetermined spacing. The spacing from the interface and the width of the groove are determined such that reflection at a boundary between the first optical waveguide and the second optical waveguide is weakened. A semiconductor board may be disposed at a boundary between the first optical waveguide and the second optical waveguide. In this case, the width of the groove and the thickness of the semiconductor board are determined such that light reflected off an interface between the first optical waveguide and the groove is weakened by light reflected from an interface between the groove and the semiconductor board, and by light reflected from an interface between the semiconductor board and the second optical waveguide.