An optical waveguide includes a substrate in the shape of a flat plate; lower clad that is disposed on the substrate; and a core that is disposed on the lower clad and transmits light. The optical waveguide includes a first optical waveguide and a second optical waveguide. The first optical waveguide includes a first core on the lower clad, and is disposed so as to extend along a direction in which the light travels to a first position. The second optical waveguide includes a second core on the lower clad, is disposed so as to extend along a direction in which the light travels to a second position, and has a lower relative refractive index difference than the first optical waveguide. The first optical waveguide and the second optical waveguide form, between the first position and the second position, a layer structure where the first core and the second core are disposed such that the first core is positioned a predetermined distance away from the second core in a direction perpendicular to the substrate. At least either the first optical waveguide or the second optical waveguide includes a mode coupling section and a mode conversion section. The mode coupling section includes a directional coupler to conduct the mode coupling of the first core and the second core between the first position and the second position. The mode conversion section is connected to the mode coupling section, and has a tapered core structure to adjust the mode diameter of the first core to the mode diameter of the second core.