Provided coaxially inside a first waveguide is a second waveguide, so as to form a coaxial waveguide. The first waveguide for the passage of a low-frequency band signal (f.sub.L signal) functions as the outer conductor of the coaxial waveguide while the second waveguide for the passage of a high-frequency band signal (f.sub.H signal) serves as the center conductor of the coaxial waveguide. Feeders for f.sub.L are provided so that they penetrate through the wall of the first waveguide. Feeders for f.sub.H are provided so that they penetrate through both the first and the second waveguide walls. The distance between the first f.sub.L feeder and the first f.sub.H feeder and the distance between the second f.sub.L feeder and the second f.sub.H feeder are set at approximately one quarter of the wavelength of the f.sub.L signal. The distance between the first f.sub.H feeder and a reflector surface is also set at about one quarter of the wavelength. A reflecting bar is provided inside the second waveguide and located at a position about one quarter of the wavelength away from the second f.sub.H feeder toward the reflector surface.