1. A hollow-waveguide-to-planar-waveguide transition circuit for transmitting a high-frequency signal, the hollow-waveguide-to-planar-waveguide transition circuit comprising:
a dielectric substrate having a first main surface and a second main surface which face each other in a thickness direction of the dielectric substrate;
one or more strip conductors formed on the first main surface, extending in a first in-plane direction determined in advance;
a ground conductor formed on the second main surface to face the one or more strip conductors in the thickness direction;
one or more slots formed in the ground conductor and extending in a second in-plane direction different from the first in-plane direction on the second main surface; and
a coupling conductor formed at a position to be electrically coupled with the one or more strip conductors on the first main surface, and disposed at a position facing the one or more slots in the thickness direction,
the coupling conductor having a main body portion electrically coupled with the one or more strip conductors, and having a protruding portion protruding from the main body portion in the second in-plane direction, the protruding portion being formed and facing, in the thickness direction, an end portion of the one or more slots in the second in-plane direction.

2. The hollow-waveguide-to-planar-waveguide transition
circuit according to claim 1, wherein:
a tip of the protruding portion is disposed outside the end portion of the one or more slots in the second in-plane direction as viewed from the thickness direction; and
a width of the tip of the protruding portion in the first in-plane direction is narrower than an entire width of the one or more slots in the first in-plane direction.
3. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 1, wherein the protruding portion has a tapered shape in which a width of the protruding portion in the first in-plane direction changes in a manner that gradually decreases the width of the protruding portion as a location of the width of the protruding portion changes from the main body portion toward the tip of the protruding portion.
4. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 1, wherein the protruding portion has a stair shape in which a width of the protruding portion in the first in-plane direction changes in a manner that stepwise decreases the width of the protruding portion as a location of the width of the protruding portion changes from the main body portion toward the tip of the protruding portion.
5. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 1, wherein a distance in the second in-plane direction between an edge of the end portion of the one or more slots and the tip of the protruding portion is

equal to or less than one eighth of a wavelength corresponding to a center frequency of a predetermined frequency band for use in the high-frequency signal.
6. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 1, further comprising a hollow waveguide having one end portion connected to an area containing the one or more slots in the ground conductor.
7. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 5, wherein a guide-axis direction of the hollow waveguide and the second main surface are orthogonal to each other.
8. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 1, wherein the main body portion is physically connected to the one or more strip conductors.
9. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 1, wherein the main body portion is disposed to be physically separated from the one or more strip conductors.
10. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 9, wherein:
the strip conductors includes a first strip conductor and a second strip conductor which are disposed to be separated from each other; and
the coupling conductor includes a first recessed portion

that surrounds an end portion of the first strip conductor facing the coupling conductor, and includes a second recessed portion that surrounds an end portion of the second strip conductor facing the coupling conductor.
11. The hollow-waveguide-to-planar-waveguide transition circuit according to claim 1, wherein both end portions of each of the one or more slots have respective widths larger than a width of a midportion of said each of the one or more slots.