There is provided a waveguide connecting structure, including first, second, third and fourth waveguides. A first coupling window at one of magnetic field planes of the third waveguide couples the first and third waveguides in such a manner that the electric field planes of both are in parallel. A second coupling window formed at one of the electric field planes of the third waveguide couples the second and third waveguides in such a manner that the electric field planes of the second waveguide is in parallel with the magnetic field planes of the first waveguide. A third coupling window formed at the other one of the electric field planes couples the fourth and third waveguides in such a manner that the electric field planes of the fourth waveguide is in parallel with the magnetic field planes of the first waveguide.