A waveguide circuit (1;2;3;4;5;5 1;5 2;5 3;5 4) comprising: a first waveguide tube (10;11;12) having a first cross-sectional shape to allow propagation of a TE mode; a second waveguide tube (20;21;22) disposed adjacent to the first waveguide tube (10;11;12) and having a second cross-sectional shape to allow propagation of a TE mode; and a third waveguide tube (30;31;30 1;30 2;30 3;30 4) having a tube axis perpendicular to both a tube axis of the first waveguide tube (10;11;12) and a tube axis of the second waveguide tube (20;21;22), and having a third cross-sectional shape to allow propagation of a TE mode and including an input/output end at a first end of the third waveguide tube, wherein, the first cross-sectional shape has a pair of straight-line long sides facing each other and a pair of straight-line short sides facing each other, in a plane orthogonal to the tube axis of the first waveguide tube (10;11;12), the second cross-sectional shape has a pair of straight-line long sides facing each other and a pair of straight-line short sides facing each other, in a plane orthogonal to the tube axis of the second waveguide tube (20;21;22), the first waveguide tube (10;11;12) has a pair of sidewalls which form the pair of straight-line short sides of the first cross-sectional shape, the second waveguide tube (20;21;22) has a pair of sidewalls which form the pair of straight-line short sides of the second cross-sectional shape, the pair of straight-line long sides of the second cross-sectional shape is parallel to the pair of straight-line long sides of the first cross-sectional shape, the tube axis of the second waveguide tube (20;21;22) is parallel to the tube axis of the first waveguide tube_ (10;11;12), one sidewall of the pair of sidewalls of the second waveguide tube (20;21;22) is disposed to face one sidewall of the pair of sidewalls of the first waveguide tube (10;11;12), and the third waveguide tube (30;31;30 1;30 2;30 3;30 4) includes a coupler which connects a hollow guide of the third waveguide tube (30;31;30 1;30 2;30 3;30 4) to both a hollow guide of the first waveguide tube (10;11;12) and a hollow guide of the second waveguide tube (20;21;22). a first waveguide tube (10;11;12) having a first cross-sectional shape to allow propagation of a TE mode; a second waveguide tube (20;21;22) disposed adjacent to the first waveguide tube (10;11;12) and having a second cross-sectional shape to allow propagation of a TE mode; and a third waveguide tube (30;31;30 1;30 2;30 3;30 4) having a tube axis perpendicular to both a tube axis of the first waveguide tube (10;11;12) and a tube axis of the second waveguide tube (20;21;22), and having a third cross-sectional shape to allow propagation of a TE mode and including an input/output end at a first end of the third waveguide tube, wherein, the first cross-sectional shape has a pair of straight-line long sides facing each other and a pair of straight-line short sides facing each other, in a plane orthogonal to the tube axis of the first waveguide tube (10;11;12), the second cross-sectional shape has a pair of straight-line long sides facing each other and a pair of straight-line short sides facing each other, in a plane orthogonal to the tube axis of the second waveguide tube (20;21;22), the first waveguide tube (10;11;12) has a pair of sidewalls which form the pair of straight-line short sides of the first cross-sectional shape, the second waveguide tube (20;21;22) has a pair of sidewalls which form the pair of straight-line short sides of the second cross-sectional shape, the pair of straight-line long sides of the second cross-sectional shape is parallel to the pair of straight-line long sides of the first cross-sectional shape, the tube axis of the second waveguide tube (20;21;22) is parallel to the tube axis of the first waveguide tube_ (10;11;12), one sidewall of the pair of sidewalls of the second waveguide tube (20;21;22) is disposed to face one sidewall of the pair of sidewalls of the first waveguide tube (10;11;12), and the third waveguide tube (30;31;30 1;30 2;30 3;30 4) includes a coupler which connects a hollow guide of the third waveguide tube (30;31;30 1;30 2;30 3;30 4) to both a hollow guide of the first waveguide tube (10;11;12) and a hollow guide of the second waveguide tube (20;21;22).