To provide a connection structure between a rectangular waveguide part and a differential line part, by which the rectangular waveguide part is directly connected to the differential line part.

The connection structure between the rectangular waveguide part 10 and the differential line part 20, is constituted so that the rectangular waveguide part 10 and the differential line part 20 which is arranged on an upper tube wall 11 of the rectangular waveguide part 10, and has a grounding conductor 22 on a lower surface of a dielectric layer 21 and a pair of line conductors 23a, 23b through which a differential signal is transmitted on an upper surface or inside are connected, wherein a rectangular slot 30 penetrating an upper tube wall 11 of the rectangular waveguide part 10 and the grounding conductor 22 is positioned, and a pair of penetrating conductors 24a, 24b for connecting each edge of a pair of line conductors 23a, 23b to the grounding conductor 22 adjacent to the center part of a pair of long sides 31a, 31b of the slot 30 are positioned. Since a differential signal is directly input in the rectangular waveguide part 10, the connection structure between the rectangular waveguide part 10 and the differential line part 20 is strong against noise from the outside and loss is small.