An optical waveguide including a first cladding layer; a core layer, including first and second core sections with cladding sections on sides thereof in the in-layer direction; and a second cladding layer. A refractive index distribution in the in-layer direction in the core layer, from the first core section to an adjacent cladding section, has a continuous change and a region with a first peak, a first dip, and a second peak in this order; the first peak at a position of the first core section, the second peak with a maximum value of refractive index smaller than of the first peak, at a position of the cladding section, and a portion, from the first cladding layer to the first core section, corresponded to a refractive index distribution in the layer-stacking direction, discontinuously changing at the boundary between the first cladding layer and first core section.