A waveguide/transmission line converter (1, 3, 4) configured to convert power transmitted by a waveguide (11, 31, 41) and power transmitted by a transmission line (12, 32, 42) to each other, the waveguide/transmission line converter (1, 3, 4) comprising: a dielectric substrate (13, 33, 43) arranged in a manner blocking an opening of the waveguide (11, 31, 41); a short-circuit metal layer (14, 34, 44) arranged on a surface of the dielectric substrate (13, 33, 43) and outside of the waveguide (11, 31, 41), and held at a potential, which is the same as a potential of the walls of the waveguide (11, 31, 41), - by a metal member (15, 35, 45) penetrating the dielectric substrate (13, 33, 43) along cross-sections of two wide walls of the waveguide (11, 31, 41) and not along cross-sections of two narrow walls of the waveguide (11, 31, 41); or - by a metal member (15, 35, 45) penetrating the dielectric substrate (13, 33, 43) 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 (11, 31, 41); and a matching element (17, 37, 47) arranged on a surface of the dielectric substrate (13, 33, 43) and inside the waveguide (11, 31, 41), coupled to the transmission line (12, 32, 42), and having a resonant length in an electric field direction inside the waveguide (11, 31, 41) and in a feed power direction of the transmission line (12, 32, 42), 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 (13, 33, 43); and the transmission line (12, 32, 42) electromagnetically coupled to the matching element (17, 37, 47) via the dielectric substrate (13, 33, 43), wherein an end shape of the transmission line (12, 32, 42) is a stub provided with a cutaway portion or a slot. a dielectric substrate (13, 33, 43) arranged in a manner blocking an opening of the waveguide (11, 31, 41); a short-circuit metal layer (14, 34, 44) arranged on a surface of the dielectric substrate (13, 33, 43) and outside of the waveguide (11, 31, 41), and held at a potential, which is the same as a potential of the walls of the waveguide (11, 31, 41), - by a metal member (15, 35, 45) penetrating the dielectric substrate (13, 33, 43) along cross-sections of two wide walls of the waveguide (11, 31, 41) and not along cross-sections of two narrow walls of the waveguide (11, 31, 41); or - by a metal member (15, 35, 45) penetrating the dielectric substrate (13, 33, 43) 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 (11, 31, 41); and - by a metal member (15, 35, 45) penetrating the dielectric substrate (13, 33, 43) along cross-sections of two wide walls of the waveguide (11, 31, 41) and not along cross-sections of two narrow walls of the waveguide (11, 31, 41); or - by a metal member (15, 35, 45) penetrating the dielectric substrate (13, 33, 43) 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 (11, 31, 41); and a matching element (17, 37, 47) arranged on a surface of the dielectric substrate (13, 33, 43) and inside the waveguide (11, 31, 41), coupled to the transmission line (12, 32, 42), and having a resonant length in an electric field direction inside the waveguide (11, 31, 41) and in a feed power direction of the transmission line (12, 32, 42), 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 (13, 33, 43); and the transmission line (12, 32, 42) electromagnetically coupled to the matching element (17, 37, 47) via the dielectric substrate (13, 33, 43), wherein an end shape of the transmission line (12, 32, 42) is a stub provided with a cutaway portion or a slot.