A hollow-waveguide-to-planar-waveguide transition circuit includes: strip conductors formed on a first main surface of a dielectric substrate; a ground conductor formed on the back side, facing the strip conductors; a slot formed in the ground conductor; and a coupling conductor formed at a position to be electrically coupled with the strip conductors. The coupling conductor has: a main body portion electrically coupled with the strip conductors; and protruding portions protruding from the main body portion. The protruding portions are formed so as to face an end portion of the slot.