A waveguide directional coupler in which a cross-sectional shape of a transformation waveguide (24a-24d) perpendicular to a waveguide-axis direction is a cutout shape obtained by cutting out the two corners on one diagonal line in a quadrangle. In the cutout shape, a first area that is longer in a waveguide-width direction than in a waveguide-height direction is connected to a connection waveguide (23a-23d) or to an input/output waveguide (25a-25d). In the cutout shape, a second area that is shorter in the waveguide-width direction than in the waveguide-height direction is connected to the input/output waveguide (25a-25d) or to the connection waveguide (23a-23d).
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