A slow-wave circuit is provided with a folded waveguide and a beam hole. The beam hole is arranged between an edge and a center in the direction of width of the folded waveguide. The beam hole is preferably arranged at an edge in the direction of width of the folded waveguide, at a position that does not protrude beyond the folded waveguide. The beam hole is preferably arranged at a position separated by a prescribed distance from the edge in the direction of width of the folded waveguide.

A slow-wave circuit is provided with a folded waveguide and a beam hole. The beam hole is arranged between an edge and a center in the direction of width of the folded waveguide. The beam hole is preferably arranged at an edge in the direction of width of the folded waveguide, at a position that does not protrude beyond the folded waveguide. The beam hole is preferably arranged at a position separated by a prescribed distance from the edge in the direction of width of the folded waveguide.