A waveguide/transmission line converter configured to convert power transmitted by a waveguide and power transmitted by a transmission line to each other, the waveguide/transmission line converter comprising: a dielectric substrate arranged in a manner blocking an opening of the waveguide ; a short-circuit metal layer arranged on a surface of the dielectric substrate and outside of the waveguide , and held at a potential, which is the same as a potential of the walls of the waveguide , - by a metal member penetrating the dielectric substrate along cross-sections of two wide walls of the waveguide and not along cross-sections of two narrow walls of the waveguide ; or - by a metal member penetrating the dielectric substrate along the cross-sections of the two wide walls and a cross-section of one of the two narrow walls of the waveguide and not along a cross-section of the other of the two narrow walls of the waveguide ; and a matching element arranged on a surface of the dielectric substrate and inside the waveguide , coupled to the transmission line , and having a resonant length in an electric field direction inside the waveguide and in a feed power direction of the transmission line , wherein the resonant length is adapted to set up, as a standing wave, an electromagnetic wave having an effective wavelength in a surrounding environment of the dielectric substrate ; and the transmission line electromagnetically coupled to the matching element via the dielectric substrate , wherein an end shape of the transmission line is a stub provided with a cutaway portion or a slot. a dielectric substrate arranged in a manner blocking an opening of the waveguide ; a short-circuit metal layer arranged on a surface of the dielectric substrate and outside of the waveguide , and held at a potential, which is the same as a potential of the walls of the waveguide , - by a metal member penetrating the dielectric substrate along cross-sections of two wide walls of the waveguide and not along cross-sections of two narrow walls of the waveguide ; or - by a metal member penetrating the dielectric substrate along the cross-sections of the two wide walls and a cross-section of one of the two narrow walls of the waveguide and not along a cross-section of the other of the two narrow walls of the waveguide ; and - by a metal member penetrating the dielectric substrate along cross-sections of two wide walls of the waveguide and not along cross-sections of two narrow walls of the waveguide ; or - by a metal member penetrating the dielectric substrate along the cross-sections of the two wide walls and a cross-section of one of the two narrow walls of the waveguide and not along a cross-section of the other of the two narrow walls of the waveguide ; and a matching element arranged on a surface of the dielectric substrate and inside the waveguide , coupled to the transmission line , and having a resonant length in an electric field direction inside the waveguide and in a feed power direction of the transmission line , wherein the resonant length is adapted to set up, as a standing wave, an electromagnetic wave having an effective wavelength in a surrounding environment of the dielectric substrate ; and the transmission line electromagnetically coupled to the matching element via the dielectric substrate , wherein an end shape of the transmission line is a stub provided with a cutaway portion or a slot. <EOS>