An optical waveguide circuit comprising a plurality of first cores (203) arranged at intervals widening as they are away from the branch point or the joining point of optical signal, a clad (205) filling at least these first cores, and second cores (204) provided between the first cores and the clad and formed in the gap between the first cores in the vicinity of the branch point or the joining point while covering the first cores at least partially. Refractive index of the second core is larger than that of the clad, the boundary between the second core and the clad is smooth and the film thickness of the second core formed in the gap between the first cores is decreased as the interval of the plurality of first cores widens.