A directional coupler (1A, 1B, 1D, 100A, 100D) comprising: a first rectangular waveguide and a second rectangular waveguide that are arranged in parallel with a narrow wall (23 and 24) therebetween and that comprise two common paired wide walls (12 and 13); a coupling window that is formed in the narrow wall (23 and 24) and is configured to electromagnetically couple the first waveguide and the second waveguide; a projecting conductor that projects, at a center in a width direction of the coupling window, the width direction corresponding to the longitudinal extension of the narrow wall, from one wide wall of the paired wide walls (12 and 13) facing each other in the first waveguide and second waveguide, toward an inside of the coupling window, the projecting conductor being spaced from the other wide wall of the paired wide walls (12 and 13), characterized in that the directional coupler further comprises: paired first conductive projections (51 and 52) formed at positions that are respectively closer to two ends of the first waveguide than the coupling window is and that are plane- symmetric with respect to a symmetry plane orthogonal to the narrow wall (23 and 24) and passing through the center of the coupling window, in the narrow wall (23 and 24), the first projections (51 and 52) projecting from the narrow wall (23 and 24) toward an inside of the first waveguide; and paired second conductive projections (53 and 54) formed at positions that are respectively closer to two ends of the second waveguide than the coupling window is and that are plane- symmetric with respect to the symmetry plane, in the narrow wall (23 and 24), the second projections (53 and 54) projecting from the narrow wall (23 and 24) toward an inside of the second waveguide; wherein the first projections (51 and 52) extend from one wide wall of the paired wide walls (12 and 13) facing each other of the first waveguide toward the other wide wall, to be electrically connected to the one wide wall, and to be spaced from the other wide wall, and the second projections (53 and 54) extend from one wide wall of the paired wide walls (12 and 13) facing each other of the second waveguide toward the other wide wall, to be electrically connected to the one wide wall and to be spaced from the other wide wall. a first rectangular waveguide and a second rectangular waveguide that are arranged in parallel with a narrow wall (23 and 24) therebetween and that comprise two common paired wide walls (12 and 13); a coupling window that is formed in the narrow wall (23 and 24) and is configured to electromagnetically couple the first waveguide and the second waveguide; a projecting conductor that projects, at a center in a width direction of the coupling window, the width direction corresponding to the longitudinal extension of the narrow wall, from one wide wall of the paired wide walls (12 and 13) facing each other in the first waveguide and second waveguide, toward an inside of the coupling window, the projecting conductor being spaced from the other wide wall of the paired wide walls (12 and 13), paired first conductive projections (51 and 52) formed at positions that are respectively closer to two ends of the first waveguide than the coupling window is and that are plane- symmetric with respect to a symmetry plane orthogonal to the narrow wall (23 and 24) and passing through the center of the coupling window, in the narrow wall (23 and 24), the first projections (51 and 52) projecting from the narrow wall (23 and 24) toward an inside of the first waveguide; and paired second conductive projections (53 and 54) formed at positions that are respectively closer to two ends of the second waveguide than the coupling window is and that are plane- symmetric with respect to the symmetry plane, in the narrow wall (23 and 24), the second projections (53 and 54) projecting from the narrow wall (23 and 24) toward an inside of the second waveguide; wherein the first projections (51 and 52) extend from one wide wall of the paired wide walls (12 and 13) facing each other of the first waveguide toward the other wide wall, to be electrically connected to the one wide wall, and to be spaced from the other wide wall, and the second projections (53 and 54) extend from one wide wall of the paired wide walls (12 and 13) facing each other of the second waveguide toward the other wide wall, to be electrically connected to the one wide wall and to be spaced from the other wide wall.