The three-dimensional structure includes first and second waveguides, a resonator, a first region. The waveguides cause light to propagate therein in a first guide mode. The second waveguide and the first region are mutually connected such that at least part of the light propagating in the second waveguide is coupled with light propagating in the first region in a guide mode different from the first guide mode. Light present in the resonator is coupled with the lights respectively propagating in the waveguides. A condition of -1.ltoreq.cos(.phi.)<0 is satisfied, where .phi. represents a phase difference between one of first and second lights whose intensity is higher than that of the other and third light. The first light is light reflected at a connection region between the first waveguide and the resonator. The second and third lights are lights coupled with the light present in the resonator and propagate in the waveguides.