A waveguide circuit (1) includes a first waveguide tube (10), a second waveguide tube (20), and a third waveguide tube (30). The first waveguide tube (10), the second waveguide tube (20), and the third waveguide tube (30) have cross-sectional shapes to allow propagation of TE modes. The tube axis of the second waveguide tube (20) is parallel to the tube axis of the first waveguide tube (10). One of the narrow sidewalls of the second waveguide tube (20) faces a narrow sidewall (10s) of the first waveguide tube (10). The third waveguide tube (30) includes a coupler that connects a hollow guide of the third waveguide tube (30) to a hollow guide of the first waveguide tube (10) and a hollow guide of the second waveguide tube (20).