An optical waveguide includes a core and a clad. The core includes first to third propagation parts, and a coupling part that couples the first to third propagation parts together. The first propagation part has a first incidence end face on which part of incident light is incident, and a first emission part that emits first propagation light. The second propagation part has a second incidence end face on which another part of the incident light is incident, and a second emission part that emits second propagation light. In the coupling part, a first light wave resulting from the first propagation light and a second light wave resulting from the second propagation light occur, and the first and second light waves interfere with each other to generate third propagation light to be emitted from a third emission part. The third propagation part propagates the third propagation light.