The present invention provides an optical waveguide which comprises a core 4 for incident light and a core 5 for outgoing light, wherein the width of the core 5 for outgoing light is more than 1.5 times that of the core 4 for incident light. This optical waveguide permits the multiplexing and/or demultiplexing of light rays even when the wavelengths thereof undergo changes within the range of about 10 nm. In addition, the present invention further provides an optical multiplexer-demultiplexer which comprises the combination of an optical waveguide serving as an optical path and a diffraction grating for demultiplexing and focusing light, wherein the absolute value of the difference between the refractive index (nTE) of the core layer of the optical waveguide in the direction parallel to the plane of the film and that (nTM) of the core layer thereof in the direction perpendicular to the plane of the film is not more than 0.007 at the wavelength used. This optical multiplexer-demultiplexer permits the control of the wavelength fluctuation observed for the outputted signal light dependent on the polarization direction of the incident signal light to not more than 5 nm by the reduction of the birefringence index of the core layer of the optical waveguide to a level of not more than 0.007.