In an optical branching unit according to the invention, the cross section shape in an area of a front stage to branch to two output waveguides in the optical branch is a rectangle or a non-circle with different vertical-to-horizontal ratio, including an area that the length along the surface of the substrate is defined to be shorter than the length in the direction orthogonal to the surface of a substrate, the cross section at the output ends of the output waveguide is a rectangle or a non-circle with a different vertical-to-horizontal ratio, where the length along the surface of the substrate is defined to be longer than the length in the direction orthogonal to the surface of the substrate.