A slot array comprises a rectangular waveguide G having a rectangular sectional shape, and a power feeder means 5 connected to the rectangular waveguide at a power feed opening 4. A plurality of wave radiation slots 1a is formed in one of the metallic plates forming the longer sides of the rectangular sectional shape. The width of the rectangular waveguide is more than four times as large as the wavelength in the space, and the height of the rectangular waveguide is more than one-half of that wavelength. Power is fed to the opening 4 in the form of a plane wave. <IMAGE>
