A signal converter configured to convert a signal between a substrate unit and a hollow waveguide includes a substrate unit, including a first conductor layer formed on one face of a dielectric substrate, and a second conductor layer formed on another face of the dielectric substrate, a plurality of conduction units that penetrate the dielectric substrate and provide conduction between the first conductor layer and the second conductor layer, an dielectric waveguide formed by the dielectric substrate, the first and second conductor layers, and the conduction units, and a conversion unit that converts the signal between the hollow waveguide and the dielectric waveguide, the conversion unit including a conductor patch having a separator region between itself and the first conductor layer, with the conductor patch being disposed on the substrate unit within an aperture of the hollow waveguide.