A directional coupler for separately detecting a travelling wave and a reflected wave, the directional coupler comprising: a waveguide having a wall surface and a tube axis, and configured to propagate the travelling wave and the reflected wave, the tube axis being the center axis of the waveguide in a width direction of the waveguide; an opening disposed in the wall surface of the waveguide and including: a first elongated hole; and a second elongated hole, the first elongated hole and the second elongated hole being disposed at a position that fails to intersect the tube axis of the waveguide in a plan view, the first elongated hole and the second elongated hole crossing each other at an opening-cross portion; and a coupling line disposed outside the waveguide and including: a first transmission line including a first intersecting-line portion; and a second transmission line including a second intersecting-line portion, wherein, in a plan view, the first intersecting-line portion extends, from one end of the tube axis, away from the tube axis as approaching a perpendicular line, and intersects the first elongated hole at a position farther away from the tube axis than the opening-cross portion is, the perpendicular line being orthogonal to the tube axis and passing through the opening-cross portion at which the first elongated hole and the second elongated hole intersect each other; in a plan view, the second intersecting-line portion extends, from another end of the tube axis, away from the tube axis as approaching the perpendicular line, and intersects the second elongated hole at a position farther away from the tube axis than the opening-cross portion is; and one end of the first intersecting-line portion is coupled to one end of the second intersecting-line portion at a position, in a plan view, out of a region in which the opening is disposed, characterized in that: the coupling line includes a plurality of straight-line portions (13aa, 13ab, 13ba, 13bb) including: the first intersecting-line portion; and the second intersecting-line portion; and, of the plurality of straight-line portions (13aa, 13ab, 13ba, 13bb), two straight-line portions adjacent to each other are coupled so as to make an obtuse angle. a waveguide having a wall surface and a tube axis, and configured to propagate the travelling wave and the reflected wave, the tube axis being the center axis of the waveguide in a width direction of the waveguide; an opening disposed in the wall surface of the waveguide and including: a first elongated hole; and a second elongated hole, the first elongated hole and the second elongated hole being disposed at a position that fails to intersect the tube axis of the waveguide in a plan view, the first elongated hole and the second elongated hole crossing each other at an opening-cross portion; and a first elongated hole; and a second elongated hole, the first elongated hole and the second elongated hole being disposed at a position that fails to intersect the tube axis of the waveguide in a plan view, the first elongated hole and the second elongated hole crossing each other at an opening-cross portion; and a coupling line disposed outside the waveguide and including: a first transmission line including a first intersecting-line portion; and a second transmission line including a second intersecting-line portion, a first transmission line including a first intersecting-line portion; and a second transmission line including a second intersecting-line portion, wherein, in a plan view, the first intersecting-line portion extends, from one end of the tube axis, away from the tube axis as approaching a perpendicular line, and intersects the first elongated hole at a position farther away from the tube axis than the opening-cross portion is, the perpendicular line being orthogonal to the tube axis and passing through the opening-cross portion at which the first elongated hole and the second elongated hole intersect each other; in a plan view, the second intersecting-line portion extends, from another end of the tube axis, away from the tube axis as approaching the perpendicular line, and intersects the second elongated hole at a position farther away from the tube axis than the opening-cross portion is; and one end of the first intersecting-line portion is coupled to one end of the second intersecting-line portion at a position, in a plan view, out of a region in which the opening is disposed, characterized in that: the coupling line includes a plurality of straight-line portions (13aa, 13ab, 13ba, 13bb) including: the first intersecting-line portion; and the second intersecting-line portion; and, of the plurality of straight-line portions (13aa, 13ab, 13ba, 13bb), two straight-line portions adjacent to each other are coupled so as to make an obtuse angle. the coupling line includes a plurality of straight-line portions (13aa, 13ab, 13ba, 13bb) including: the first intersecting-line portion; and the second intersecting-line portion; and, the first intersecting-line portion; and the second intersecting-line portion; and, of the plurality of straight-line portions (13aa, 13ab, 13ba, 13bb), two straight-line portions adjacent to each other are coupled so as to make an obtuse angle.